tupdesc: simplify assert in equalTupleDescs()

Hi,

While reading the code, I saw these assertions in equalTupleDescs():
```
CompactAttribute *cattr1 = TupleDescCompactAttr(tupdesc1, i);
CompactAttribute *cattr2 = TupleDescCompactAttr(tupdesc2, i);

Assert(cattr1->attnullability != ATTNULLABLE_UNKNOWN);
Assert((cattr1->attnullability == ATTNULLABLE_UNKNOWN) ==
(cattr2->attnullability == ATTNULLABLE_UNKNOWN));

```

The first assertion already guarantees that cattr1->attnullability is not ATTNULLABLE_UNKNOWN, so in the second one the expression cattr1->attnullability == ATTNULLABLE_UNKNOWN will always be false, That means the second assertion is effectively just checking that cattr2->attnullability is also not ATTNULLABLE_UNKNOWN.

So the current code is correct, but it feels a bit harder to read than necessary. This patch just simplifies the second assertion in a direct way.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/