(Bug 5238) Return an empty hashref instead of undef

This makes it so callers don't have to care whether the POST was valid
or not, when looking up whether something might be in $r->post_args
(Bug 5238) Return an empty Hash::MultiValue in case POST was invalid
Merge pull request #554 from afuna/bug5238-no-post_args

(Bug 5238) Return an empty hashref instead of undef