D7 Default text formats are not saved properly without accompanying values

I try to set a default text format to a field (Long text) but it's not saved if i don't set a default value for the field.

1. I edit the field
2. I set the Text processing to "Filtered text (user selects text format)"
3. I set "Text format" to a another text format
4. Then i save

If i also set a default value for my field, my text format is set as default
But if i leave the default value of my field, it's not saved.

The function "text_field_is_empty (field/modules/text/text.module - line 51-)" is called. This function check only the value for the field, not the format. It return true and the function "_field_filter_items (field/field.module - line 51-)" unset this item including my text format.

If i change the function function text_field_is_empty

function text_field_is_empty($item, $field) {
  if ((!isset($item['value']) || $item['value'] === '') && (!isset($item['format']) || $item['format'] === '')) {
    return !isset($item['summary']) || $item['summary'] === '';
  }
  return FALSE;
}

to

function text_field_is_empty($item, $field) {
  if (!isset($item['value']) || $item['value'] === '')) {
    return !isset($item['summary']) || $item['summary'] === '';
  }
  return FALSE;
}

my text format is set as default event i don't enter a default value for my field.

But i'm not sure it's the good way to fix this issue.