[Foundation] The operators in the various NS[Mutable][Ordered]Set[<T>] classes are inconsistent.

[rolfbjarne]

Task:

• Look at the various NSSet classes in src/Foundation/
• Make sure the operators treat null/empty/non-empty input the same way. In particular:
  • Addition operators must return null if and only if both operands are null
  • Subtraction operators must return null if the first operand is null, irrespective of what the second operator is.
  • The operators must have [return: NotNullIfNotNull] attributes when applicable.
• The xml docs should be matching as much as possible.
• Make sure all operators are well tested with unit tests (in the tests/monotouch-test/Foundation directory)

Assign to Copilot once #17285 is complete, to avoid conflicting results.