postgres_fdw: Remove useless ternary expression.
There is no case where we would call pgfdw_exec_cleanup_query or
pgfdw_exec_cleanup_query_{begin,end} with a NULL query string, so this
expression is pointless; remove it and instead add to the latter
functions an assertion ensuring the given query string is not NULL.
Thinko in commit
815d61fcd.
Discussion: https://postgr.es/m/CAPmGK14mm%2B%3DUjyjoWj_Hu7c%2BQqX-058RFfF%2BqOkcMZ_Nj52v-A%40mail.gmail.com