ArgumentNullException when using a minimal Json file with the JsonConfigurationProvider

[BrennanConroy]

Description

Using a minimal Json file {}
with the JsonConfigurationProvider fails with:

System.ArgumentNullException : Value cannot be null. (Parameter 'key')
   at System.Collections.Generic.SortedDictionary`2.set_Item(TKey key, TValue value)
   at Microsoft.Extensions.Configuration.Json.JsonConfigurationFileParser.VisitElement(JsonElement element)
   at Microsoft.Extensions.Configuration.Json.JsonConfigurationFileParser.ParseStream(Stream input)
   at Microsoft.Extensions.Configuration.Json.JsonConfigurationFileParser.Parse(Stream input)
   at Microsoft.Extensions.Configuration.Json.JsonConfigurationProvider.Load(Stream stream)
   at Microsoft.Extensions.Configuration.FileConfigurationProvider.Load(Boolean reload)
--- End of stack trace from previous location ---
   at Microsoft.Extensions.Configuration.FileConfigurationProvider.HandleException(ExceptionDispatchInfo info)
   at Microsoft.Extensions.Configuration.FileConfigurationProvider.Load(Boolean reload)
   at Microsoft.Extensions.Configuration.FileConfigurationProvider.Load()
   at Microsoft.Extensions.Configuration.ConfigurationRoot..ctor(IList`1 providers)
   at Microsoft.Extensions.Configuration.ConfigurationBuilder.Build()

This is because _currentPath will be null and try to set the null to null in the new code path.

runtime/src/libraries/Microsoft.Extensions.Configuration.Json/src/JsonConfigurationFileParser.cs

Line 60 in a52f237

_data[_currentPath] = null;

Regression?

Yes, caused by #40829

cc @maryamariyan

This is blocking the Runtime -> AspNetCore dependency update. We can skip our failing tests to workaround, but ideally this will be fixed ASAP.

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[maryamariyan]

got a repro.. working on it.