Catch broken `mac_ver()` (#11396)
A user reported a homebrew Python that would raise an exception in the
interpreter probing script because `platform.mac_ver()` returned `('',
('', '', ''), '')` on his installation due to
https://github.com/Homebrew/homebrew-core/issues/206778
This is easy enough to catch and show a proper error message instead of
the Python backtrace.
