Low: libcrmcommon: Improve prefix spacing in XML logging functions

In cases where the prefix may be empty, include a space after the prefix
only if the prefix is not empty.

A NULL prefix may be impossible in some places now; pcmk__s() is used
anyway for future-proofing and in preparation for an upcoming refactor.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>