Fix PHP warning when updating issue with date CF

On PHP 8, when updating an issue in a project where a date custom field
is linked, and the field's value is not set (empty string), Mantis
throws a Warning:

Uncaught TypeError: date(): Argument #2 ($timestamp) must be of type
?int, string given

This fixes the issue by setting the date to 0 if it is not numeric,
prior to calling print_date_selection_set().

Fixes #27928

(cherry picked from commit c09fb41)

core/cfdefs/cfdef_standard.php

@@ -497,6 +497,9 @@ function cfdef_input_textarea( array $p_field_def, $p_custom_field_value, $p_req
  * @return void
  */
 function cfdef_input_date( $p_field_def, $p_custom_field_value, $p_required = '' ) {
+	if( !is_numeric( $p_custom_field_value ) ) {
+		$p_custom_field_value = 0;
+	}
 	print_date_selection_set( 'custom_field_' . $p_field_def['id'], config_get( 'short_date_format' ), $p_custom_field_value, false, true, 0, 0, 'input-sm', $p_required );
 }