**Note**: These changes are generated by hand. Review with care.

This CL changes the `absl_internal` implementations of `AsciiStrToLower()` /
`AsciiStrToUpper()` to allow `src` to be null. This can, in fact, happen if the
`string_view` passed to the public API is empty, and the implementations handle
it correctly. I have added comments noting that `src` is allowed to be null
iff the size is zero.

`-Wnonnull` diagnoses cases where a `nullptr` literal is passed to a parameter
annotated nonnull, or where `nullptr` is returned from a function whose return
type is annotated nonnull.

`-Wnullability` diagnoses cases where nullability annotations conflict, for
example between the declaration and definition of a function.

PiperOrigin-RevId: 673846759
Change-Id: I6cf3490ce13837eba9814156c420598000ecc596