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 (D113148).
CJ -- please mention "bugprone-stringview-nullptr" explicitly and explain that it will cover those features, etc. so that the description will be clear on its own. Thanks!
Slightly prefer an early return here:
I'm in 2 minds about this. This diagnostic is a good fit for this warning and shouldn't be removed. Likewise duplicate diagnostics from different checks is annoying. Maybe a better path forward would be to suppress the diagnostic if the bugprone-stringview-nullptr check is enabled. These kinds of intercheck dependencies are found in modernize-prefer-member-init check IIRC. You can just copy the impl from there.