Indexing exception is thrown for a disabled eventhub function with null connection string

If you have an empty string configured for an eventhub connection string in a dotnet precompiled function app disabled via the `[disable]` attribute, the function runtime throws indexing error.

#### Investigative information

#### Repro steps
1. Create a v3 dotnet precompiled function app
2. Add an eventhub trigger function
3. Add `[disable]` attribute to the function
4. Set the corresponding connection string setting to empty in the `local.settings.json` file
5. Press F5 to run

One thing to note is that this behavior is specific for eventhub trigger functions only.

#### Expected behavior
Function runtime should not throw the indexing error.

#### Actual behavior
Function runtime throws indexing error and complains about invalid connection string

#### Known workarounds
- Set a valid connection string

OR

- Set a dummy connection string
`"Endpoint=sb://placeHolder.servicebus.windows.net/;SharedAccessKeyName=RootManageSharedAccessKey;SharedAccessKey=placeHolder"`

OR

- Disable indexing on the eventhub function by adding all the non-disabled functions to allowed function list by configuring functions property in the host.json. Below is the sample configuration.

```
"Functions" : [ "ActiveFuction1", "ActiveFunction2"]

Reproduce the v3 .NET precompiled function scenario using the listed steps, focusing on an Event Hubs trigger with an empty connection string and the [disable] attribute. Trace the runtime's indexing path to determine why the disabled function still validates the connection string; done means indexing completes without an error while the function remains disabled.

Written by the indexing model from the issue text.