pgsql: Make pg_partition_tree return no rows on unsupported and undefin

Make pg_partition_tree return no rows on unsupported and undefined objects

The function was tweaked so as it returned one row full of NULLs when
working on an unsupported relkind or an undefined object as of cc53123,
and after discussion with Amit and Álvaro it looks more natural to make
it return no rows.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera, Amit Langote
Discussion: /messages/by-id/20190227184808.GA17357@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0f3cdf873e7d81fbf61965e13685595c8a23c322

Modified Files
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src/backend/utils/adt/partitionfuncs.c | 6 +++---
src/test/regress/expected/partition_info.out | 9 +++------
2 files changed, 6 insertions(+), 9 deletions(-)