pgsql: Fix pg_stat_ssl.clientdn

Fix pg_stat_ssl.clientdn

Return null if there is no client certificate.  This is how it has
always been documented, but in reality it returned an empty string.

Reviewed-by: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Discussion: https://www.postgresql.org/message-id/flat/398754d8-6bb5-c5cf-e7b8-22e5f0983caf@2ndquadrant.com/

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e77cfa54d700557ea700d47454c9e570f20f1841

Modified Files
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src/backend/utils/adt/pgstatfuncs.c | 5 ++++-
src/test/ssl/t/001_ssltests.pl      | 2 +-
2 files changed, 5 insertions(+), 2 deletions(-)