queue: Fix STAILQ_ASSERT_EMPTY()

The 'while' part corresponding to the 'do' was missing.

Did not notice the problem as later commits using it have been stashed
and never reworked up to now, and it is currently unused in the tree.

While here, fix spacing after the '#define' in the !(_KERNEL &&
INVARIANS) part.

Fixes: 34740937f7a4 ("queue: New debug macros for STAILQ")
MFC after: 1 minute
Sponsored by: The FreeBSD Foundation