Dependency Injection not instantiating instances when using function.json rather than attributes

I've created an Azure Function in C#, and attempting to utilize the dependency injection feature while also defining the Function parameters with a `function.json` file rather than the .net attributes so that I can use route parameters in the Function methods as described in [Customize the HTTP Endpoint](https://docs.microsoft.com/en-us/azure/azure-functions/functions-bindings-http-webhook-trigger?tabs=csharp#customize-the-http-endpoint).

When I use the `HttpTrigger` and `FunctionName` attributes, the dependency injection works as expected. However, when I remove the attributes, and manually define the `function.json` file, the runtime throws an exception:
```
System.Private.CoreLib: Exception while executing function: MyClass.myFunction. Anonymously Hosted DynamicMethods Assembly: Object reference not set to an instance of an object.
```

To be sure, I've copied the automatically generated `function.json` file when using the attributes in order to ensure I hadn't miss-typed a value somewhere (removed the `configurationSource` field and ensured my written `function.json` file is in the `output` directory)

Versions and other information:
`.csproj` file sample:
```

netcoreapp3.1
v3







```

Functions Core Tools:
```
$ func --version
3.0.3331
```

I haven't seen a sample of using dependency injection with route parameters, so I may have missed something in the setup. My `Startup.cs` file matches the example in the [Dependency Injection docs](https://docs.microsoft.com/en-us/azure/azure-functions/functions-dotnet-dependency-injection).

Happy to provide any other information, or a reproducible example repo if needed.

Start with the reported .csproj package versions, Startup.cs, and the manually written function.json, then reproduce with Functions Core Tools 3.0.3331. Compare the attribute-generated function.json with the manual file and trace the null-reference exception during invocation. Done means dependency injection works with route parameters when using the manual function.json configuration.

Written by the indexing model from the issue text.