Handle null strings in I18nInput.FillArray

Until now i always had the translations be empty strings if some columns
did not have the full translation, but this is going too far on the
non-NULL policy: surely they have a translations, but we do not know it
yet; this is the exact type of situations NULL values are for.

Besides the philosophical distinction, having empty strings instead of
NULLs is less practical, because i no longer can user coalesce() to
retrieve the default language text in case the translation for that
particular field is not available, even if the row for a locale exists.

In time i will change all _i18n relations, but for now only these from
campsite follow the “new policy”.

pkg/form/input.go

@@ -70,7 +70,12 @@ func (input I18nInput) FillArray(array database.RecordArray) error {
 		if err != nil {
 			return err
 		}
-		input[tag.String()].Val = el.Fields[1].Get().(string)
+		val := el.Fields[1].Get()
+		if val == nil {
+			input[tag.String()].Val = ""
+		} else {
+			input[tag.String()].Val = val.(string)
+		}
 	}
 	return nil
 }