Research meaning of null keyword in MSBuild

[ghogen]

User feedback (on the docs):
Please document the use of the "null" keyword in MSBuild function calls. There is already an example of it at GetRegistryValueFromView but no specification of what it means in the MSBuild language.- As an argument of a property function, null is a case-insensitive keyword that means a null reference. It is not the same as 'null', which instead means a four-character string.- In MSBuild conditions however, null means the same as 'null'.- Does it work in item functions too?
https://github.com/dotnet/msbuild/pull/8745/files#r1547744498
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However this issue is about discovering the full story as an engineering research project, so that we can then write the docs.