Set Nullable in the solution

[rkargMsft]

Nullable analysis should be enabled on the Solution.

In order to not break everything, the following should get done:

1. Enable Nullable in the Solution. Existing patterns for common project settings should be followed.
2. For every file that has warnings now for nullable usage, disable nullable analysis at the top of that file and restore it at the end

It is very important that no attempt is made to address any nullable warnings when the setting is enabled as that will be handled in future work. It does not get done in this set of work.

This way, all new files will need to properly handle nullable variable but the existing code can be incrementally addressed as time allows after the above work is completed.

[rkargMsft]

@copilot I choose you!