subr_prf: Don't write kern.boot_tag if it's empty

This change allows one to set kern.boot_tag="" and not get a blank line
preceding other boot messages. While this isn't super critical- blank lines
are easy to filter out both mentally and in processing dmesg later- it
allows for a mode of operation that matches previous behavior.

I intend to MFC this whole series to stable/11 by the end of the month with
boot_tag empty by default to make this effectively a nop in the stable
branch.