Fix: Guard PG result with nil check

[TonyCTHsu]

closes #3507

What does this PR do?

It is unclear how PG is yielding or returning nil instead of PG::Result.

I cannot reproduce the issue and here's a list what I have tried:

1. I tried sending non-SELECT query (DELETE FROM...), PG::Result is returned
2. I tried closing the connection object in the block, it raises exception
3. I tried assigning the result object in the block and accessing it afterward, the PG::Result has a cleared status.
4. I have tried clearing the result object in the block, it raises exception

Hence this PR implements:

1. Guards result with nil check
2. Tag with result before the block invokation