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

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

core/string/node_path.cpp

@@ -368,7 +368,7 @@ NodePath::NodePath(const String &p_path) {
 		for (int i = from; i <= path.length(); i++) {
 			if (path[i] == ':' || path[i] == 0) {
 				String str = path.substr(from, i - from);
-				if (str == "") {
+				if (str.is_empty()) {
 					if (path[i] == 0) {
 						continue; // Allow end-of-path :
 					}

core/string/string_builder.cpp

@@ -33,7 +33,7 @@
 #include <string.h>
 
 StringBuilder &StringBuilder::append(const String &p_string) {
-	if (p_string == String()) {
+	if (p_string.is_empty()) {
 		return *this;
 	}
 

core/string/string_name.cpp

@@ -310,7 +310,7 @@ StringName::StringName(const String &p_name, bool p_static) {
 
 	ERR_FAIL_COND(!configured);
 
-	if (p_name == String()) {
+	if (p_name.is_empty()) {
 		return;
 	}
 
@@ -434,7 +434,7 @@ StringName StringName::search(const char32_t *p_name) {
 }
 
 StringName StringName::search(const String &p_name) {
-	ERR_FAIL_COND_V(p_name == "", StringName());
+	ERR_FAIL_COND_V(p_name.is_empty(), StringName());
 
 	MutexLock lock(mutex);
 

core/string/translation.cpp

@@ -1287,7 +1287,7 @@ bool TranslationServer::_load_translations(const String &p_from) {
 void TranslationServer::setup() {
 	String test = GLOBAL_DEF("internationalization/locale/test", "");
 	test = test.strip_edges();
-	if (test != "") {
+	if (!test.is_empty()) {
 		set_locale(test);
 	} else {
 		set_locale(OS::get_singleton()->get_locale());

core/string/ustring.cpp

@@ -4283,7 +4283,7 @@ bool String::is_valid_filename() const {
 		return false;
 	}
 
-	if (stripped == String()) {
+	if (stripped.is_empty()) {
 		return false;
 	}
 
@@ -4902,7 +4902,7 @@ String DTRN(const String &p_text, const String &p_text_plural, int p_n, const St
 String RTR(const String &p_text, const String &p_context) {
 	if (TranslationServer::get_singleton()) {
 		String rtr = TranslationServer::get_singleton()->tool_translate(p_text, p_context);
-		if (rtr == String() || rtr == p_text) {
+		if (rtr.is_empty() || rtr == p_text) {
 			return TranslationServer::get_singleton()->translate(p_text, p_context);
 		} else {
 			return rtr;
@@ -4915,7 +4915,7 @@ String RTR(const String &p_text, const String &p_context) {
 String RTRN(const String &p_text, const String &p_text_plural, int p_n, const String &p_context) {
 	if (TranslationServer::get_singleton()) {
 		String rtr = TranslationServer::get_singleton()->tool_translate_plural(p_text, p_text_plural, p_n, p_context);
-		if (rtr == String() || rtr == p_text || rtr == p_text_plural) {
+		if (rtr.is_empty() || rtr == p_text || rtr == p_text_plural) {
 			return TranslationServer::get_singleton()->translate_plural(p_text, p_text_plural, p_n, p_context);
 		} else {
 			return rtr;