build: luci.mk: gracefully handle missing or unversioned po subdirectories

Fixes: #3911
Fixes: 9d8e99f9b build: gracefully handle non-Git source trees
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

luci.mk

@@ -65,9 +65,13 @@ define findrev
   $(shell \
     if git log -1 >/dev/null 2>/dev/null; then \
       set -- $$(git log -1 --format="%ct %h" --abbrev=7 -- '$(if $(1),:(exclude))po'); \
+      if [ -n "$$1" ]; then
         secs="$$(($$1 % 86400))"; \
         yday="$$(date --utc --date="@$$1" "+%y.%j")"; \
         printf 'git-%s.%05d-%s' "$$yday" "$$secs" "$$2"; \
+      else \
+        echo "unknown"; \
+      fi; \
     else \
       ts=$$(find . -type f $(if $(1),-not) -path './po/*' -printf '%T@\n' 2>/dev/null | sort -rn | head -n1 | cut -d. -f1); \
       if [ -n "$$ts" ]; then \