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);
 }