Fixed most SUF keys matching all units if given "" as a value

This was a regression from a6decea. We only want to ignore
keys if they're not provided. An empty key ("") should match nothing.

Fixes #1405 (a specific bug about id="" matching all units).

changelog

@@ -96,6 +96,8 @@ Version 1.13.11:
    * A missing [event] name= key will now raise a WML error instead of being
      silently ignored.
    * Units hidden with [hide_unit] will remain hidden after reloading saves.
+   * Fixed regression where most SUF keys would match all units if given "" as
+     a value.
  * Miscellaneous and bug fixes:
    * Suggested save file names now use spaces instead of underscores.
    * Fixed crash after canceling add-on download (bug #2203)

src/units/filter.cpp

@@ -280,7 +280,7 @@ void unit_filter_compound::create_child(const vconfig& c, F func)
 template<typename C, typename F>
 void unit_filter_compound::create_attribute(const config::attribute_value v, C conv, F func)
 {
-	if(v.empty()) {
+	if(v.blank()) {
 	}
 	else if(v.apply_visitor(contains_dollar_visitor())) {
 		children_.emplace_back(new unit_filter_attribute_literal<C, F>(std::move(v.str()), std::move(conv), std::move(func)));