pgsql: Remove redundant null pointer checks before PQclear and PQconnin

Remove redundant null pointer checks before PQclear and PQconninfoFree

These functions already had the free()-like behavior of handling null
pointers as a no-op. But it wasn't documented, so add it explicitly
to the documentation, too.

Discussion: https://www.postgresql.org/message-id/flat/dac5d2d0-98f5-94d9-8e69-46da2413593d%40enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5faef9d582012433db9ad05af27a77bd591508e1

Modified Files
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contrib/dblink/dblink.c | 6 ++----
contrib/postgres_fdw/postgres_fdw.c | 24 ++++++++----------------
doc/src/sgml/libpq.sgml | 5 +++++
src/bin/pg_basebackup/streamutil.c | 6 ++----
src/bin/pg_dump/pg_dumpall.c | 3 +--
src/bin/psql/command.c | 3 +--
src/bin/psql/common.c | 3 +--
src/bin/psql/describe.c | 3 +--
src/fe_utils/query_utils.c | 3 +--
src/interfaces/ecpg/ecpglib/descriptor.c | 3 +--
src/interfaces/ecpg/ecpglib/execute.c | 3 +--
src/interfaces/libpq/fe-connect.c | 6 ++----
src/interfaces/libpq/fe-exec.c | 9 +++------
13 files changed, 29 insertions(+), 48 deletions(-)