[BUG] routePrefix cannot be non-empty for FastAPI to work with ASGI Function on Python v2

### Investigative information
###### Please provide the following:

- Timestamp: 2023-08-28T22:49:33+00:00
- Function App name:
- Function name(s) (as appropriate):
- Core Tools version: 4.0.5274

### Repro steps
###### Provide the steps required to reproduce the problem:

1. Start a new Azure Function app with the following command: `func init <> --worker-runtime python --model V2`
2. In `function_app.py`, create a new ASGI Function App with the following code:
```
import azure.functions as func
import logging

from fastapi import FastAPI

api = FastAPI()

@api.get("/")
def root():
return {"message": "Hello World"}

@api.get("/healthcheck")
def healthcheck():
return 1

app = func.AsgiFunctionApp(
app=api,
http_auth_level=func.AuthLevel.ANONYMOUS
)
```
3. Add `fastapi` to `requirements.txt` (currently on 0.103.0)
4. Add the following to `host.json`:
```
"extensions": {
"http": {
"routePrefix": "api"
}
}
```
6. Run `func start` using core tools from cli

### Expected behavior
###### Provide a description of the expected behavior.

Should be able to access `http://localhost:7071/api` with no issue.

### Actual behavior
###### Provide a description of the actual behavior observed.
Following error is received:
```
❯ func start
Found Python version 3.10.11 (py).

Azure Functions Core Tools
Core Tools Version: 4.0.5274 Commit hash: N/A (64-bit)
Function Runtime Version: 4.23.0.20886

[2023-08-28T23:02:24.316Z] Worker process started and initialized.
[2023-08-28T23:02:24.385Z] A host error has occurred during startup operation 'a87fe4da-9c2c-4dd7-958f-c9b73021ea2c'.
[2023-08-28T23:02:24.387Z] Microsoft.AspNetCore.Routing: An error occurred while creating the route with name 'http_app_func' and template 'api//{*route}'. Microsoft.AspNetCore.Routing: The route template separator character '/' cannot appear consecutively. It must be separated by either a parameter or a literal value. (Parameter 'routeTemplate'). Microsoft.AspNetCore.Routing: The route template separator character '/' cannot appear consecutively. It must be separated by either a parameter or a literal value.
[2023-08-28T23:02:24.399Z] Failed to stop host instance '4b8b0863-dcb1-4a41-9b70-0beaad496108'.
[2023-08-28T23:02:24.401Z] Microsoft.Azure.WebJobs.Host: The host has not yet started.
Value cannot be null. (Parameter 'provider')
[2023-08-28T23:02:24.409Z] Host startup operation has been canceled
```

### Known workarounds
###### Provide a description of any known workarounds.

Only workaround is to make the `routePrefix` in `host.json` an empty string.

```
{
"version": "2.0",
"logging": {
"applicationInsights": {
"samplingSettings": {
"isEnabled": true,
"excludedTypes": "Request"
}
}
},
"extensionBundle": {
"id": "Microsoft.Azure.Functions.ExtensionBundle",
"version": "[4.*, 5.0.0)"
},
"extensions": {
"http": {
"routePrefix": ""
}
}
}
```

### Contents of the requirements.txt file:
###### Provide the requirements.txt file to help us find out module related issues.

```
azure-functions
fastapi==0.103.0
```

### Related information
###### Provide any related information

* Links to source
* Bindings used

Source

```python
# __init__.py

import azure.functions as func
import logging

from fastapi import FastAPI

api = FastAPI()

@api.get("/")
def root():
return {"message": "Hello World"}

@api.get("/healthcheck")
def healthcheck():
return 1

app = func.AsgiFunctionApp(
app=api,
http_auth_level=func.AuthLevel.ANONYMOUS
)
```

```txt
azure-functions
fastapi==0.103.0
```