Add nullptr check for String TLV data with zero length (project-chip#…

…36921)

* Add nullptr check for String TLV data with zero length

* Compare len instead of buf

src/lib/support/BufferWriter.cpp

@@ -30,7 +30,7 @@ BufferWriter & BufferWriter::Put(const void * buf, size_t len)
 {
     size_t available = Available();
 
-    if (available > 0)
+    if (available > 0 && len > 0)
     {
         memmove(mBuf + mNeeded, buf, available < len ? available : len);
     }