AK: StringBuilder with 0 initial capacity shouldn't build null String

With 0 initial capacity, we don't allocate an underlying ByteBuffer
for the StringBuilder, which would then lead to a null String() being
returned from to_string().

This patch makes sure we always build a valid String.

AK/StringBuilder.cpp

@@ -91,6 +91,8 @@ ByteBuffer StringBuilder::to_byte_buffer() const
 
 String StringBuilder::to_string() const
 {
+    if (is_empty())
+        return String::empty();
     return String((const char*)m_buffer.data(), m_length);
 }
 

AK/Tests/TestString.cpp

@@ -199,4 +199,13 @@ TEST_CASE(split)
     EXPECT_EQ(parts[2].characters()[3], '\0');
 }
 
+TEST_CASE(builder_zero_initial_capacity)
+{
+   StringBuilder builder(0);
+   builder.append("");
+   auto built = builder.build();
+   EXPECT_EQ(built.is_null(), false);
+   EXPECT_EQ(built.length(), 0u);
+}
+
 TEST_MAIN(String)