Moved initialization into else block

implicit_return_defaults is not properly initialized. I am implementing a MERGE statement (inherits from Executable and ClauseElement) that is neither INSERT nor UPDATE. This causes access to the uninitialized variable in line 1979.

"SELECT FIRST n ROWS" using a bound parameter (only firebird has both),
combined with column-level subqueries
which also feature "limit" as well as "positional" bound parameters
(e.g. qmark style) would erroneously assign the subquery-level positions
before that of the enclosing SELECT, thus returning parameters which
are out of order. Fixes #3038

Found using: https://github.com/intgr/topy

a join condition that is strictly from a single column to itself,
translated through some kind of SQL function or expression.  This
is kind of experimental, but the first proof of concept is a
"materialized path" join condition where a path string is compared
to itself using "like".   The :meth:`.Operators.like` operator has
also been added to the list of valid operators to use in a primaryjoin
condition.  fixes #3029

specify a :func:`.text` expression as the target; the index no longer
needs to have a table-bound column present if the index is to be
manually added to the table, either via inline declaration or via
:meth:`.Table.append_constraint`. fixes #3028

adding an argument for a construct not previously included for any
special arguments would fail. fixes #3024

feature from :ticket:`1068` would not apply quoting rules to the
label name as rendered in the ORDER BY.
fix #3020, re: #1068

import namespace, which was removed at the beginning of 0.9.