MDL-61194 assignfeedback_editpdf: Check if we can convert online text

This patch adds a check to make sure there is a converter available that
will be able to handle html->pdf conversion before adding onlinetext
submissions to the list of files to combine.

mod/assign/feedback/editpdf/classes/document_services.php

@@ -188,7 +188,7 @@ protected static function list_compatible_submission_files_for_attempt($assignme
                         } else if ($convertedfile = $converter->start_conversion($file, 'pdf')) {
                             $files[$filename] = $convertedfile;
                         }
-                    } else {
+                    } else if ($converter->can_convert_format_to('html', 'pdf')) {
                         // Create a tmp stored_file from this html string.
                         $file = reset($file);
                         // Strip image tags, because they will not be resolvable.