Only show non empty messages in feedback.

Some problems prevent messages by overriding the default message with
the string consisting of a single space.  See the sample problem
`tutorials/sample-problems/Algebra/EquationImplicitFunction.pg` for
example.  If those messages are used, then the feedback button shows the
dot indicating the presence of a message even if there really isn't a
message.  To fix this, the answer messages are only shown if they do not
consist entirely of whitespace.

Note that I do not consider this a good practice for problem authors to
employ.  Even the previous attempts table in WeBWorK 2.18 or before
would have shown the message column in this case even though there are
no actual messages.  Furthermore, in most cases there are better ways to
prevent these messages from occuring.  In the case of the sample problem
mentioned above, I believe that there is something wrong with
`paserImplicitEquation.pl` macro's `createPoints` method that causes the
message to be displayed for some answers where it should not be, and
that is a bug that should be looked into. So I would be completely fine
with not accepting this pull request.

macros/PG.pl

@@ -1198,9 +1198,9 @@ sub ENDDOCUMENT {
 					'button',
 					type  => 'button',
 					class => "ww-feedback-btn btn btn-sm $options{btnClass} $options{btnAddClass}"
-						. ($rh_envir->{showMessages} && $ansHash->{ans_message} ? ' with-message' : ''),
+						. ($rh_envir->{showMessages} && $ansHash->{ans_message} =~ /\S/ ? ' with-message' : ''),
 					'aria-label' => (
-						$rh_envir->{showMessages} && $ansHash->{ans_message}
+						$rh_envir->{showMessages} && $ansHash->{ans_message} =~ /\S/
 						? maketext('[_1] with message', $options{resultTitle})
 						: $options{resultTitle}
 					),
@@ -1238,7 +1238,7 @@ sub ENDDOCUMENT {
 									class => 'card',
 									sub {
 										(
-											$rh_envir->{showMessages} && $ansHash->{ans_message}
+											$rh_envir->{showMessages} && $ansHash->{ans_message} =~ /\S/
 											? $feedbackLine->(
 												'', $ansHash->{ans_message} =~ s/\n/<br>/gr,
 												'feedback-message'