cel/cel_pgsql.c: Use the 'SEP' macro when appending a column name

When appending a column name to the sql buffer, the predicate, "if first is
non-null, use empty string; else, use comma", is identical to the 'SEP' macro
definition. Since they are the same, this patch replaces the redundant
predicate statement with the 'SEP' macro.

Change-Id: Ib8b6138b06a48381723108a05ab8752cb8700509