Enable '#nullable' context

[henrygab]

Enabling non-nullable reference types requires explicit update to the .csproj file. This is done for backwards-compatibility.

Current compiler warnings

Three files are currently trying to use the non-nullable reference types feature, without the feature actually being enabled. The compiler emits the following warning for each of Enumerate/BlockEnumerator.cs,
Enumerate/MergeEnumerator.cs, and
Enumerate/TableEnumerator.cs:

warning CS8632: The annotation for nullable reference types should only 
                be used in code within a '#nullable' annotations context. 

See C# documentation for details.
See also relevant stackoverflow thread.

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Caveats (click to expand)

For most cases, enabling nullable annotations context should be straightforward. However, there are some functions where special annotations are required. These cases are more fully described by the corresponding documentation on migration strategies.

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If interested, I am happy to work on preparing a PR to enable this ... just let me know.