BUG/MEDIUM: sample: urlp can't match an empty value

BUG/MEDIUM: sample: urlp can't match an empty value

Currently urlp fetching samples were able to find parameters with an empty
value, but the return code depended on the value length. The final result was
that acls using urlp couldn't match empty values.

Example of acl which always returned "false":
  acl MATCH_EMPTY urlp(foo) -m len 0

The fix consists in unconditionally return 1 when the parameter is found.

This fix must be backported to 1.6 and 1.5.