PartitionKeyData->partattrs, refactor some 0 to InvalidAttrNumber

hi.

if PartitionKeyData->partattrs is 0, then it means that partition key is
expression, else it's column reference.

we can change from
if (key->partattrs[i] == 0)
to
if (key->partattrs[i] == InvalidAttrNumber)

the reason I can think of is to improve grepability.
numeric value 0 is too common to reliably locate relevant code.

what do you think?

--
jian
https://www.enterprisedb.com/