share/mk: Use :ts,:[*] to stringify TAG_ARGS

This avoids a situation where we sometimes prepend an empty tag,
leading to arguments like "-T ,package=foo".

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D51785

(cherry picked from commit 399faa7d3ae8938d73d58c9224123a71a2096947)