log: print log entry terminator even if the message is empty

log: print log entry terminator even if the message is empty

This eliminates a special case in the show_log() function, to help
simplify the terminator semantics.  Now show_log() always prints a
newline after the log entry when use_terminator is set, even if the log
message is empty.

This change should only affect the --pretty=tformat output, since that
was the only way to trigger this special case.

Signed-off-by: Adam Simpkins <adam@adamsimpkins.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>