NullReferenceException in GetFallbackWorkerConifg

Message: Object reference not set to an instance of an object.
Exception type: System.NullReferenceException
Failed method: Microsoft.Azure.WebJobs.Script.FunctionMetadataManager.GetFallbackWorkerConfig

This problem appears only after we migrate to isolated dotnet worker. And it happens a lot (4.5k exceptions in one day). Seems doesn't impact our functionality(?), but I still would like to point it out.

Callstack:
```
System.NullReferenceException:
at Microsoft.Azure.WebJobs.Script.FunctionMetadataManager.GetFallbackWorkerConfig (Microsoft.Azure.WebJobs.Script, Version=4.22.0.0, Culture=neutral, PublicKeyToken=null: /_/src/WebJobs.Script/Host/FunctionMetadataManager.cs:128)
at Microsoft.Azure.WebJobs.Script.FunctionMetadataManager.LoadFunctionMetadata (Microsoft.Azure.WebJobs.Script, Version=4.22.0.0, Culture=neutral, PublicKeyToken=null: /_/src/WebJobs.Script/Host/FunctionMetadataManager.cs:136)
at Microsoft.Azure.WebJobs.Script.FunctionMetadataManager.GetFunctionMetadata (Microsoft.Azure.WebJobs.Script, Version=4.22.0.0, Culture=neutral, PublicKeyToken=null: /_/src/WebJobs.Script/Host/FunctionMetadataManager.cs:89)
at Microsoft.Azure.WebJobs.Script.FunctionMetadataManager.b__17_0 (Microsoft.Azure.WebJobs.Script, Version=4.22.0.0, Culture=neutral, PublicKeyToken=null: /_/src/WebJobs.Script/Host/FunctionMetadataManager.cs:71)
at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd (System.Collections.Concurrent, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a)
at Microsoft.Azure.WebJobs.Script.FunctionMetadataManager.TryGetFunctionMetadata (Microsoft.Azure.WebJobs.Script, Version=4.22.0.0, Culture=neutral, PublicKeyToken=null: /_/src/WebJobs.Script/Host/FunctionMetadataManager.cs:69)
at Microsoft.Azure.WebJobs.Script.WebHost.Diagnostics.FunctionInstanceLogger.StartFunction (Microsoft.Azure.WebJobs.Script.WebHost, Version=4.22.0.0, Culture=neutral, PublicKeyToken=null: /_/src/WebJobs.Script.WebHost/Diagnostics/FunctionInstanceLogger.cs:65)
at Microsoft.Azure.WebJobs.Script.WebHost.Diagnostics.FunctionInstanceLogger.AddAsync (Microsoft.Azure.WebJobs.Script.WebHost, Version=4.22.0.0, Culture=neutral, PublicKeyToken=null: /_/src/WebJobs.Script.WebHost/Diagnostics/FunctionInstanceLogger.cs:53)
at Microsoft.Azure.WebJobs.Host.Loggers.CompositeFunctionEventCollector+d__2.MoveNext (Microsoft.Azure.WebJobs.Host, Version=3.0.37.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35: D:\a\_work\1\s\src\Microsoft.Azure.WebJobs.Host\Loggers\CompositeFunctionEventCollector.cs:21)
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw (System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
at System.Runtime.CompilerServices.TaskAwaiter.GetResult (System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor+d__18.MoveNext (Microsoft.Azure.WebJobs.Host, Version=3.0.37.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35: D:\a\_work\1\s\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:106)
```

PackageRefence:
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