pgsql: Fix missing space before WHERE in GRAPH_TABLE deparse

Fix missing space before WHERE in GRAPH_TABLE deparse

get_graph_pattern_def() emitted the pattern-level WHERE keyword as
"WHERE " with no leading space, so reverse-parsing produced output
like "(o IS orders)WHERE (...)". The element-level WHERE deparse in
get_path_pattern_expr_def() already prepends a separating space; the
pattern-level branch was inconsistent with it. Emit " WHERE " to
match.

The output still re-parses to the same tree, so this is cosmetic.

For test coverage, add a whole-pattern WHERE clause to the existing
customers_us view, which is already reverse-parsed with
pg_get_viewdef().

Author: Dhruv Chauhan <chauhandhruv351(at)gmail(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CANWwWcpHb0h7tg6otRnL-FV83jwQpAiyw1bhvv8T78kpwZ-0ow%40mail.gmail.com

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d2ac26eb9b1e8ad19fd526efb0ffff4fae5ed56f

Modified Files
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src/backend/utils/adt/ruleutils.c | 2 +-
src/test/regress/expected/graph_table.out | 21 +++++++++++----------
src/test/regress/sql/graph_table.sql | 5 +++--
3 files changed, 15 insertions(+), 13 deletions(-)