analyze: return earlier when there is no path to be prepended

Follow-up for 2f6181ad4d.

Addresses the post-merge comment:
https://github.com/systemd/systemd/pull/30172#discussion_r1404209035

src/analyze/analyze-verify-util.c

@@ -98,13 +98,13 @@ int verify_set_unit_path(char **filenames) {
                         return r;
         }
 
+        if (strv_isempty(ans))
+                return 0;
+
         joined = strv_join(strv_uniq(ans), ":");
         if (!joined)
                 return -ENOMEM;
 
-        if (isempty(joined))
-                return 0;
-
         /* First, prepend our directories. Second, if some path was specified, use that, and
          * otherwise use the defaults. Any duplicates will be filtered out in path-lookup.c.
          * Treat explicit empty path to mean that nothing should be appended. */