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/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