Retries don't work using a session adapter with empty prefix ("")

When using a Requests `Session` with an `HTTPAdapter` with `urllib3.util.Retry` configuration supplied, when the adapter is bound to an empty string prefix (`""`), the retries don't execute.

## Expected Result

This was an attempt to set up a session where every request in the session would have the retries executed, regardless of the prefix. A glance at the Requests `Session` source code seems to indicate this should work... the empty string can be a key in the `OrderedDict` storing the `Session`'s adapters, and checking `url.startswith("")` should return `True`. However...

## Actual Result

The retry configuration is ignored, and you're given a `Response` object with the erroneous error code instead of a raised `requests.exceptions.RetryError`. When binding the Adapter to the session with an actual prefix (for example, `"https://"`), the Retry behavior is observed.

## Reproduction Steps

The below script can be run to show the behavior, and below that I've posted the output I receive from it.

```python
from requests import Session
from requests.adapters import HTTPAdapter
from requests.exceptions import RetryError
from urllib3.util import Retry
​
​
class LogRetry(Retry):
    def __init__(self, *args, **kwargs):
        total_retries = kwargs["total"]
        print(f"Retry {total_retries} initiating...")
        super().__init__(*args, **kwargs)
​
​
def test_case(retry_clazz, prefix):
    s = Session()
    s.mount(
        prefix,
        HTTPAdapter(
            max_retries=retry_clazz(
                total=3,
                backoff_factor=0.1,
                status_forcelist=[500],
                allowed_methods={"POST"},
            )
        ),
    )
    try:
        print(f"Trying with retry class {retry_clazz}, prefix: {prefix}")
        s.post("https://httpbin.org/status/500", {})
    except RetryError:
        print("Got the retry error, hooray!")
    else:
        print("Didn't get the retry error, oh no!")
​
​
test_case(Retry, "https://")
test_case(Retry, "")
test_case(LogRetry, "https://")
test_case(LogRetry, "")
```

Results:

```
Trying with retry class <class 'urllib3.util.retry.Retry'>, prefix: https://
Got the retry error, hooray!
Trying with retry class <class 'urllib3.util.retry.Retry'>, prefix: 
Didn't get the retry error, oh no!
Retry 3 initiating...
Trying with retry class <class '__main__.LogRetry'>, prefix: https://
Retry 2 initiating...
Retry 1 initiating...
Retry 0 initiating...
Retry -1 initiating...
Got the retry error, hooray!
Retry 3 initiating...
Trying with retry class <class '__main__.LogRetry'>, prefix: 
Didn't get the retry error, oh no!
```

## System Information

    $ python -m requests.help

```json
{
  "chardet": {
    "version": "5.2.0"
  },
  "charset_normalizer": {
    "version": null
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.4"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.11.5"
  },
  "platform": {
    "release": "6.5.3-arch1-1",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.28.2"
  },
  "system_ssl": {
    "version": "30100030"
  },
  "urllib3": {
    "version": "1.26.15"
  },
  "using_charset_normalizer": false,
  "using_pyopenssl": false
}             
```

<!-- This command is only available on Requests v2.16.4 and greater. Otherwise,
please provide some basic information about your system (Python version,
operating system, &c). -->


We use the longest match and document that. So if you do not remove the other adapters this doesn't work and it is the intended behavior 

Ah, you must be referring to the implicit adapters defined on `Session` instantiation at https://github.com/psf/requests/blob/881281250f74549f560408e5546d95a8cd73ce28/src/requests/sessions.py#L448. So that means in order to make this work, you'd have to do something like...

```python
s = Session()
s.adapters.clear()
s.mount("", HTTPAdapter(.....))
```

That makes sense, thanks. This is one area of requests where the documentation could be much clearer. Would it be appropriate to continue extending the Advanced Usage section I added in https://github.com/psf/requests/pull/6258 (https://requests.readthedocs.io/en/latest/user/advanced/#example-automatic-retries) with some of these additional details/gotchas? Or would it be better to start a different place in the docs for more in-depth documentation?