fix(strings): non fatal errors for [NULL] safety in vim_snprintf

context: vim_snprintf provides a "portable" replacement for a system
vsnprintf(). However we rely on system vsnprintf() in places. This
is quite arbitrary. I want to use system vsnprintf() only, and
reduce the machinery in string.c for typval printf only, which will
delet a lot of duplicated code.

The biggest hurdle here is not "portably" (we only support sane c
runtimes) but the following discrepancy: standard libc printf()
considers NULL args to %s to be undefined behavior strictly
while vim_snprintf() _defacto_ allows this by replacement to "[NULL]"

IMO, this should be seen as "graceful" error handling (nicer than crashing
on the user), not a valid means to have the string "[NULL]" intentionally
be shoved in the users face. more robust code should explicitly
check for NULL and replace it with a _appropriate_ fallback for
the situation, depending on what a NULL string actually means in the
context (unnamed buffer? anonymous namespace? global augroup?)

soo, this PR provides the most _gently_ incremental nudge towards
considering this case again as an error one could possible imagine.
If sanitizers are complied in, this minor incursion gets printed
into the sanitizers' log as a "report-and-continue" error with a
traceback. This doesn't annoy the end user or "stop the world"
in the test suite, but the CI will see the reported error and
fail the build, just like other non-fatal CI errors.

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