Fix strftime/1 and strflocaltime/1 to allow an empty result (#3670)

These functions failed with "unknown system failure" whenever strftime()
returned 0 for a non-empty format, but 0 also means the format expands to
an empty string, as %p does in a locale without AM/PM.

Prepend a character to the format and trim it from the result, so that 0
can only mean the result is too long. Now 0 | strflocaltime("%p") gives
"" in fr_FR.UTF-8, and a result that is still too long fails with
"result too long" error instead. The size limit is unchanged.

tests/shtest

@@ -779,6 +779,15 @@ EOF
   fi
 fi
 
+# strflocaltime/1 allows a format that expands to an empty string
+l=fr_FR.UTF-8
+if ! $msys && ! $mingw && [ -z "$(LC_ALL=$l date +%p 2>/dev/null)" ]; then
+  if ! out=$(LC_ALL=$l $VALGRIND $Q $JQ -n '0 | strflocaltime("%p")' 2>&1) || [ "$out" != '""' ]; then
+    echo "strflocaltime(\"%p\") in $l should output an empty string but got: $out"
+    exit 1
+  fi
+fi
+
 # Test syntax error messages of multiline filter (we can't test in jq.test)
 if $VALGRIND $Q $JQ -n '[
   try if .