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

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.