CString.isEmpty() for String.utf8() returns true when String has a single character

https://bugs.webkit.org/show_bug.cgi?id=303428

Reviewed by Darin Adler.

isEmpty was accounting for a null terminator in the CStringBuffer length but it is not like that.

Test: Tools/TestWebKitAPI/Tests/WTF/CString.cpp
* Source/WTF/wtf/text/CString.h:
* Tools/TestWebKitAPI/Tests/WTF/CString.cpp:
(TEST(WTF, CStringEmptyRegularConstructor)):
(TEST(WTF, CStringOnyByte)):

Canonical link: https://commits.webkit.org/303840@main

Author: calvaris

Source/WTF/wtf/text/CString.h

@@ -95,7 +95,7 @@ class CString final {
     size_t length() const;
 
     bool isNull() const { return !m_buffer; }
-    bool isEmpty() const { return isNull() || m_buffer->length() <= 1; }
+    bool isEmpty() const { return isNull() || !m_buffer->length(); }
     bool isSafeToSendToAnotherThread() const;
 
     CStringBuffer* buffer() const LIFETIME_BOUND { return m_buffer.get(); }

Tools/TestWebKitAPI/Tests/WTF/CString.cpp

@@ -87,6 +87,23 @@ TEST(WTF, CStringEmptyRegularConstructor)
     ASSERT_STREQ(referenceString, stringWithLength.data());
 }
 
+TEST(WTF, CStringOneByte)
+{
+    const char* referenceString = "W";
+
+    CString string(referenceString);
+    ASSERT_FALSE(string.isNull());
+    ASSERT_FALSE(string.isEmpty());
+    ASSERT_EQ(string.length(), strlen(referenceString));
+    ASSERT_STREQ(referenceString, string.data());
+
+    CString stringWithLength(std::span { referenceString, 1 });
+    ASSERT_FALSE(stringWithLength.isNull());
+    ASSERT_FALSE(stringWithLength.isEmpty());
+    ASSERT_EQ(stringWithLength.length(), strlen(referenceString));
+    ASSERT_STREQ(referenceString, stringWithLength.data());
+}
+
 TEST(WTF, CStringUninitializedConstructor)
 {
     std::span<char> buffer;