Bug: PostgresContainer().get_connection_url(driver=None) does not function as documented

**Describe the bug**



`get_connection_url(driver=None)` should return a connection string starting `postgresql://...`, but actually returns `postgresql+None://...`



This is not inline with the documented intention:

```

        If a driver is set in the constructor (defaults to psycopg2!), the URL will contain the

        driver. The optional driver argument to :code:`get_connection_url` overwrites the constructor

        set value. Pass :code:`driver=None` to get URLs without a driver.

```



**To Reproduce**



```bash

$ python <<EOF

from testcontainers.postgres import PostgresContainer

postgres = PostgresContainer("postgres:16")

postgres.start()

print(f"{postgres.get_connection_url(driver=None)=}")

EOF



WARNING:root:DOCKER_AUTH_CONFIG is experimental, see testcontainers/testcontainers-python#566

Pulling image testcontainers/ryuk:0.7.0

INFO:testcontainers.core.container:Pulling image testcontainers/ryuk:0.7.0

Container started: 2d9d0ae2c1f5

INFO:testcontainers.core.container:Container started: 2d9d0ae2c1f5

Waiting for container <Container: 2d9d0ae2c1f5> with image testcontainers/ryuk:0.7.0 to be ready ...

INFO:testcontainers.core.waiting_utils:Waiting for container <Container: 2d9d0ae2c1f5> with image testcontainers/ryuk:0.7.0 to be ready ...

Pulling image postgres:16

INFO:testcontainers.core.container:Pulling image postgres:16

Container started: 2dba2ba8e9ec

INFO:testcontainers.core.container:Container started: 2dba2ba8e9ec

Waiting for container <Container: 2dba2ba8e9ec> with image postgres:16 to be ready ...

INFO:testcontainers.core.waiting_utils:Waiting for container <Container: 2dba2ba8e9ec> with image postgres:16 to be ready ...

Waiting for container <Container: 2dba2ba8e9ec> with image postgres:16 to be ready ...

INFO:testcontainers.core.waiting_utils:Waiting for container <Container: 2dba2ba8e9ec> with image postgres:16 to be ready ...

postgres.get_connection_url(driver=None)='postgresql+None://test:test@localhost:32776/test'

```



It does work if `driver=None` is passed to the constructor. I find this unintuitive.



```bash

python <<EOF 

from testcontainers.postgres import PostgresContainer

postgres = PostgresContainer("postgres:16", driver=None)

postgres.start()

print(f"{postgres.get_connection_url()=}")

EOF



WARNING:root:DOCKER_AUTH_CONFIG is experimental, see testcontainers/testcontainers-python#566

Pulling image testcontainers/ryuk:0.7.0

INFO:testcontainers.core.container:Pulling image testcontainers/ryuk:0.7.0

Container started: 4a0e020f0691

INFO:testcontainers.core.container:Container started: 4a0e020f0691

Waiting for container <Container: 4a0e020f0691> with image testcontainers/ryuk:0.7.0 to be ready ...

INFO:testcontainers.core.waiting_utils:Waiting for container <Container: 4a0e020f0691> with image testcontainers/ryuk:0.7.0 to be ready ...

Pulling image postgres:16

INFO:testcontainers.core.container:Pulling image postgres:16

Container started: ce239a341dab

INFO:testcontainers.core.container:Container started: ce239a341dab

Waiting for container <Container: ce239a341dab> with image postgres:16 to be ready ...

INFO:testcontainers.core.waiting_utils:Waiting for container <Container: ce239a341dab> with image postgres:16 to be ready ...

Waiting for container <Container: ce239a341dab> with image postgres:16 to be ready ...

INFO:testcontainers.core.waiting_utils:Waiting for container <Container: ce239a341dab> with image postgres:16 to be ready ...

postgres.get_connection_url()='postgresql://test:test@localhost:32776/test'

```



**Runtime environment**



Provide a summary of your runtime environment. Which operating system, python version, and docker version are you using? What is the version of `testcontainers-python` you are using? You can run the following commands to get the relevant information.



```bash

# Get the operating system information (on a unix os).

$ uname -a

Darwin ip-... 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:11:08 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T8122 arm64



# Get the python version.

$ python --version

Python 3.11.9



# Get the docker version and other docker information.

$ docker info

Client:

 Version:    25.0.5

 Context:    orbstack

 Debug Mode: false

 Plugins:

  buildx: Docker Buildx (Docker Inc.)

    Version:  v0.13.1

    Path:     /Users/oliverlambson/.docker/cli-plugins/docker-buildx

  compose: Docker Compose (Docker Inc.)

    Version:  v2.24.5

    Path:     /Users/oliverlambson/.docker/cli-plugins/docker-compose



# Get all python packages.

$ pip freeze

certifi==2024.2.2

charset-normalizer==3.3.2

docker==7.1.0

idna==3.7

requests==2.32.2

testcontainers==4.5.0

typing_extensions==4.12.0

urllib3==2.2.1

wrapt==1.16.0



```



This is either a documentation issue (it does work if `driver=None` is passed to the constructor) or it can be handled in code, so that the driver can be overridden by the method in [this line](https://github.com/testcontainers/testcontainers-python/blob/111bd094428b83233d7eca693d94e10b34ee8ae8/modules/postgres/testcontainers/postgres/__init__.py#L82):



```diff

-         driver_str = self.driver if driver is _UNSET else f"+{driver}"

+         driver_str = f"+{driver}" if driver not in [_UNSET, None] else self.driver

```

[]