Fix missing space before WHERE in `GRAPH_TABLE` deparse

Hi,

*Issue:*
The `get_graph_pattern_def()` function omits a leading space before the
WHERE keyword when a GRAPH_TABLE graph pattern includes a WHERE clause.
This produces incorrectly formatted SQL during reverse-parsing. e.g.

... (o IS orders)WHERE (c.address = 'US'::text) COLUMNS ...

Notice the missing space between ) and WHERE.

*Root Cause:*
The pattern-level WHERE deparse doesn't add a leading space, while the
element-level WHERE deparse already does. This inconsistency creates
malformed output.

*Solution:*
The patch changes the pattern-level branch to emit " WHERE " (with leading
space) instead of "WHERE ", matching the element-level behavior. Although
the SQL still re-parses correctly (making this cosmetic), the formatted
output is now correct.

Also adds a regression test for whole-pattern WHERE clauses. make check is
green. Patch is against master.

Thanks,
Dhruv Chauhan