Ensure fileName is not empty in WebworkWebservice.

When rendering a problem via WebworkWebservice, only use $rh->{fileName}
if it is defined and not empty.  This ensures that the
$ProblemRecord->source_file is set so images and other assets can be
found relative to the original file name when previewing, checking,
or showing correct answers in the PGProblemEditor rendered problem.

This fixes openwebwork#2556.

lib/WebworkWebservice/RenderProblem.pm

@@ -189,13 +189,16 @@ async sub renderProblem {
 	my $r_problem_source;
 	if ($rh->{problemSource}) {
 		$r_problem_source = \(decode_utf8_base64($rh->{problemSource}) =~ tr/\r/\n/r);
-		$problemRecord->source_file(defined $rh->{fileName} ? $rh->{fileName} : $rh->{sourceFilePath});
+		$problemRecord->source_file(defined $rh->{fileName}
+				&& $rh->{fileName} ? $rh->{fileName} : $rh->{sourceFilePath});
 	} elsif ($rh->{rawProblemSource}) {
 		$r_problem_source = \$rh->{rawProblemSource};
-		$problemRecord->source_file(defined $rh->{fileName} ? $rh->{fileName} : $rh->{sourceFilePath});
+		$problemRecord->source_file(defined $rh->{fileName}
+				&& $rh->{fileName} ? $rh->{fileName} : $rh->{sourceFilePath});
 	} elsif ($rh->{uriEncodedProblemSource}) {
 		$r_problem_source = \(url_unescape($rh->{uriEncodedProblemSource}));
-		$problemRecord->source_file(defined $rh->{fileName} ? $rh->{fileName} : $rh->{sourceFilePath});
+		$problemRecord->source_file(defined $rh->{fileName}
+				&& $rh->{fileName} ? $rh->{fileName} : $rh->{sourceFilePath});
 	} elsif (defined $rh->{sourceFilePath} && $rh->{sourceFilePath} =~ /\S/) {
 		$problemRecord->source_file($rh->{sourceFilePath});
 		$r_problem_source = \(readFile($ce->{courseDirs}{templates} . '/' . $rh->{sourceFilePath}));