umpf: Fix topicless tag creation

Calling umpf tag on an useries without topics aborts with the following
error message before creating the final octopus commit and the tags:

  /usr/bin/umpf: line 1129: .git/umpf/topics: No such file or directory

Fix this by allowing an empty topic list (or non-existent
.git/umpf/topics state) during umpf tag.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

umpf

@@ -1279,6 +1279,7 @@ Upstream-Status: Inappropriate [autogenerated]"
 
 create_tag() {
 	local args tagname parents tree topic
+	local -a topics
 
 	if [ ! -e "${STATE}/tag" ]; then
 		return
@@ -1290,7 +1291,8 @@ create_tag() {
 	fi
 
 	tree="$(${GIT} log -1 --format="%T")"
-	for topic in $(<"${STATE}/topics"); do
+	mapfile -t topics < <(cat ${STATE}/topics 2>/dev/null)
+	for topic in "${topics[@]}"; do
 		parents="${parents} -p ${topic}"
 	done
 	commit=$(${GIT} commit-tree "${tree}" -p HEAD ${parents} -m "${tagname}" -F "${STATE}/series.next")