check-toolchain-clean.sh: workaround stray rebuilds

It seems, that there are currently some unhandled corner cases in which
`.toolchain_build_ver` results in empty file and thus forcing rebuilds,
even if the toolchain was build correctly just a few moments ago. Until
proper fix is found, workaround that by checking for this corner case
and simply populate `.toolchain_build_ver` file.

While at it, improve the UX and display version mismatch, so it's more
clear what has forced the rebuild:

 "Toolchain build version changed (11.2.0-1 != ), running make targetclean"

References: https://gitlab.com/ynezz/openwrt/-/jobs/2133332533/raw
Signed-off-by: Petr Štetiar <ynezz@true.cz>

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