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

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

tests/core/io/test_file_access.h

@@ -53,7 +53,7 @@ TEST_CASE("[FileAccess] CSV read") {
 	REQUIRE(row2.size() == 3);
 	CHECK(row2[0] == "GOOD_EVENING");
 	CHECK(row2[1] == "Good Evening");
-	CHECK(row2[2] == ""); // Use case: not yet translated!
+	CHECK(row2[2].is_empty()); // Use case: not yet translated!
 	// https://github.com/godotengine/godot/issues/44269
 	CHECK_MESSAGE(row2[2] != "\"", "Should not parse empty string as a single double quote.");
 

tests/core/math/test_math.cpp

@@ -312,7 +312,7 @@ public:
 					}
 				}
 
-				if (name != String()) {
+				if (!name.is_empty()) {
 					namespace_stack[at_level] = name;
 				}
 

tests/core/string/test_string.h

@@ -157,10 +157,10 @@ TEST_CASE("[String] Invalid UTF8") {
 	String s;
 	bool err = s.parse_utf8((const char *)u8str);
 	CHECK(err);
-	CHECK(s == String());
+	CHECK(s.is_empty());
 
 	CharString cs = (const char *)u8str;
-	CHECK(String::utf8(cs) == String());
+	CHECK(String::utf8(cs).is_empty());
 	ERR_PRINT_ON
 }
 
@@ -170,10 +170,10 @@ TEST_CASE("[String] Invalid UTF16") {
 	String s;
 	bool err = s.parse_utf16(u16str);
 	CHECK(err);
-	CHECK(s == String());
+	CHECK(s.is_empty());
 
 	Char16String cs = u16str;
-	CHECK(String::utf16(cs) == String());
+	CHECK(String::utf16(cs).is_empty());
 	ERR_PRINT_ON
 }
 
@@ -1439,6 +1439,24 @@ TEST_CASE("[String] Variant ptr indexed set") {
 
 	CHECK_EQ(s, String("azcd"));
 }
+
+TEST_CASE("[Stress][String] Empty via ' == String()'") {
+	for (int i = 0; i < 100000; ++i) {
+		String str = "Hello World!";
+		if (str.is_empty()) {
+			continue;
+		}
+	}
+}
+
+TEST_CASE("[Stress][String] Empty via `is_empty()`") {
+	for (int i = 0; i < 100000; ++i) {
+		String str = "Hello World!";
+		if (str.is_empty()) {
+			continue;
+		}
+	}
+}
 } // namespace TestString
 
 #endif // TEST_STRING_H

tests/scene/test_code_edit.h

@@ -3235,7 +3235,7 @@ TEST_CASE("[SceneTree][CodeEdit] Backspace delete") {
 	code_edit->insert_text_at_caret("line 1\nline 2\nline 3");
 	code_edit->select_all();
 	code_edit->backspace();
-	CHECK(code_edit->get_text() == "");
+	CHECK(code_edit->get_text().is_empty());
 
 	/* Backspace at the beginning without selection has no effect. */
 	code_edit->set_text("");