tree-wide: use strv_isempty() instead of strv_length() == 0

It's a lot faster in many cases, since it's O(1) rather than O(n).

src/tmpfiles/tmpfiles.c

@@ -1788,7 +1788,7 @@ static bool should_include_path(const char *path) {
 
         /* no matches, so we should include this path only if we
          * have no whitelist at all */
-        if (strv_length(arg_include_prefixes) == 0)
+        if (strv_isempty(arg_include_prefixes))
                 return true;
 
         log_debug("Entry \"%s\" does not match any include prefix, skipping.", path);