Filter string_view from the nullptr diagnosis of bugprone-string-constructor...

Filter string_view from the nullptr diagnosis of bugprone-string-constructor to prevent duplicate warnings with bugprone-stringview-nullptr

Updates the check and tests to not diagnose the null case for string_view (but retains it for string). This prevents the check from giving duplicate warnings that are caught by bugprone-stringview-nullptr ([[ https://reviews.llvm.org/D113148 | D113148 ]]).

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D114823