Revert "ovsdb-idl: Fix NULL deref reported by Coverity."

This reverts commit 68bc6f8.
The commit causes a regression in OVN scale test. ovn-northd's CPU
more than doubled for the test scenario: create and bind 12k ports.
Below are some perf data of ovn-northd when running command:
  ovn-nbctl --wait=sb sync

Before reverting this commit:
-   92.42%     0.62%  ovn-northd  ovn-northd          [.] main
   - 91.80% main
      + 68.93% ovn_db_run (inlined)
      + 22.45% ovsdb_idl_loop_commit_and_wait

After reverting this commit:
-   92.84%     0.60%  ovn-northd  ovn-northd          [.] main
   - 92.24% main
      + 92.03% ovn_db_run (inlined)

Reverting this commit avoided 22.45% of the CPU caused by
ovsdb_idl_loop_commit_and_wait().

The commit changed the logic of ovsdb_idl_txn_write__() by adding
the check "datum->keys && datum->values" before discarding unchanged
data in a transaction. However, it is normal for OVSDB clients (
such as ovn-northd) to try to set columns with same empty data
as it is before the transaction. IDL would discard these changes
and avoid sending big transactions to server (which would end up as
no-op on server side). In the ovn scale test scenario mentioned above,
each iteration of ovn-northd would send a transaction to server that
includes all rows of the huge Port_Binding table, which caused the
significant CPU increase of ovn-northd (and also the OVN SB DB server),
resulted in longer end to end latency of OVN configuration changes.

For the original problem the commit 68bc6f8 was trying to fix, it
doesn't seem to be a real problem. The NULL deref reported by
Coverity may be addressed in a future patch using a different approach,
if necessary.

Signed-off-by: Han Zhou <[email protected]>
Signed-off-by: Ilya Maximets <[email protected]>

lib/ovsdb-idl.c

@@ -4631,8 +4631,7 @@ ovsdb_idl_txn_write__(const struct ovsdb_idl_row *row_,
      * transaction only does writes of existing values, without making any real
      * changes, we will drop the whole transaction later in
      * ovsdb_idl_txn_commit().) */
-    if (datum->keys && datum->values &&
-        write_only && ovsdb_datum_equals(ovsdb_idl_read(row, column),
+    if (write_only && ovsdb_datum_equals(ovsdb_idl_read(row, column),
                                          datum, &column->type)) {
         goto discard_datum;
     }