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This handles the case where an empty bytes/bytearray/str could pass in
NULL as the str argument (with length zero). This would result in UB in
strncmp. Even though our bare-metal implementation of strncmp handles
this, best to avoid it for when we're using system strncmp.

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>