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/

src/backend/utils/adt/pgstatfuncs.c

@@ -652,7 +652,10 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 			values[20] = CStringGetTextDatum(beentry->st_sslstatus->ssl_cipher);
 			values[21] = Int32GetDatum(beentry->st_sslstatus->ssl_bits);
 			values[22] = BoolGetDatum(beentry->st_sslstatus->ssl_compression);
-			values[23] = CStringGetTextDatum(beentry->st_sslstatus->ssl_clientdn);
+			if (beentry->st_sslstatus->ssl_clientdn[0])
+				values[23] = CStringGetTextDatum(beentry->st_sslstatus->ssl_clientdn);
+			else
+				nulls[23] = true;
 		}
 		else
 		{

src/test/ssl/t/001_ssltests.pl

@@ -316,7 +316,7 @@ command_like([
 	'-c', "SELECT * FROM pg_stat_ssl WHERE pid = pg_backend_pid()"
 			 ],
 			 qr{^pid,ssl,version,cipher,bits,compression,clientdn\n
-				^\d+,t,TLSv[\d.]+,[\w-]+,\d+,f,$}mx,
+				^\d+,t,TLSv[\d.]+,[\w-]+,\d+,f,_null_$}mx,
 			 'pg_stat_ssl view without client certificate');
 
 ### Server-side tests.