When sorting by due_date, display undefined values last

Markus' original patch was modified to simplify the code and reduce
duplication, fix a couple typos in comment text and reword the
commit message.

Fixes #16259

core/filter_api.php

@@ -954,7 +954,14 @@ function filter_get_query_sort_data( &$p_filter, $p_show_sticky, $p_query_clause
 
 			# standard column
 			} else {
-				$p_query_clauses['order'][] = "$t_bug_table.$c_sort $c_dir";
+				$t_sort_col = "$t_bug_table.$c_sort";
+
+				# when sorting by due_date, always display undefined dates last
+				if( 'due_date' == $c_sort && 'ASC' == $c_dir ) {
+					$t_sort_col = "$t_sort_col = 1, $t_sort_col";
+				}
+
+				$p_query_clauses['order'][] = "$t_sort_col $c_dir";
 			}
 		}
 	}