Replace String comparisons with "", String() to is_empty()

Also:
- Adds two stress tests to test_string.h
- Changes to .empty() on std::strings

visual_script_yield_nodes.cpp

@@ -449,7 +449,7 @@ void VisualScriptYieldSignal::_validate_property(PropertyInfo &property) const {
 
 		String ml;
 		for (const String &E : mstring) {
-			if (ml != String()) {
+			if (!ml.is_empty()) {
 				ml += ",";
 			}
 			ml += E;