check-code: fix check for trailing whitespace on empty lines

It seems like the intention with c7d23b4ca4ba was to catch lines with only
one whitespace too.

contrib/check-code.py

@@ -91,7 +91,7 b' testfilters = ['
 uprefix = r"^  \$ "
 utestpats = [
   [
-    (r'^(\S|  \$ ).*(\S[ \t]+|^[ \t]+)\n', "trailing whitespace on non-output"),
+    (r'^(\S.*||  \$ .*)[ \t]\n', "trailing whitespace on non-output"),
     (uprefix + r'.*\|\s*sed[^|>\n]*\n',
      "use regex test output patterns instead of sed"),
     (uprefix + r'(true|exit 0)', "explicit zero exit unnecessary"),