Consistently use macro HeapTupleIsValid to check the validity of tuples in tablecmds.c

Hi,

While working on another patch, I find that tablecmds.c now has three
ways to check the validity of catalog tuples.
i.
if (tuple != NULL)

ii.
if (!tuple)

iii.
if (HeapTupleIsValid(tuple)

In tablecmds.c, most checks use macro HeapTupleIsValid. For
code readability,
I changed the first and the second formats to the third one, e.g., using
HeapTupleIsValid.

BTW,  I searched the other files, some files also have different ways to
check the validity of tuples.
But the attached patch only focuses on tablecmds.c

Any thoughts?

-- 
Thanks, Tender Wang