fix: request type annotation None is now correctly supported and value-check and supports "null"

[sveneberth]

Resolves #1423

• "null" is helpful if you build requests in JS have null values
• If a annotation is missing, it's param.empty but this is difference to an explicit set None type annotation and needs to processed separately as empty
• The other "simple types" do a type-cast and would raise a ValueError in case of invalid data, but None was always taken, regardless of the value?!

Here some examples of definition which had some issues before and are now working as expected:

    @exposed
    def my_method(
        self,
        *,
        test0: None | db.Key | str = None,
        test1: None = None,
        test2: None | str = None,
        test3: str | None = None,
        test4=None,
        test5: int = None,
    ):

Of course test1: None = None is a little bit impractical, but since it's a valid syntax it should also work in this way and accept only None, "None" or "null". But test4 has no type annotation and accepts any value as-is.

Providing "None" for test2 would result into None, while "None" for test3 would result into "None" (due to the order).

However, my initially problem parameter test0: None | db.Key | str = None, is now working as expected: I can provide explicit "None" or "null" and it's no longer always None.