msxml3: Mirror the NULL checking logic (Coverity).

dlls/msxml3/mxwriter.c

@@ -1202,7 +1202,7 @@ static void mxwriter_write_starttag(mxwriter *writer, const WCHAR *qname, int le
     write_node_indent(writer);
 
     write_output_buffer(writer->buffer, ltW, 1);
-    write_output_buffer(writer->buffer, qname, len);
+    write_output_buffer(writer->buffer, qname ? qname : emptyW, qname ? len : 0);
     writer_inc_indent(writer);
 }