Mark set_repmgrd_pid() as "RETURNS NULL ON NULL INPUT"

When unsetting the PID, we'll want to set the pidfile to NULL rather
than an empty string.

dbutils.c

@@ -1920,7 +1920,7 @@ repmgrd_set_pid(PGconn *conn, pid_t repmgrd_pid, const char *pidfile)
 	else
 	{
 		appendPQExpBufferStr(&query,
-							 " '')");
+							 " NULL)");
 	}
 
 	res = PQexec(conn, query.data);

repmgr--4.4--4.5.sql

@@ -1,3 +1,5 @@
 -- complain if script is sourced in psql, rather than via CREATE EXTENSION
 \echo Use "CREATE EXTENSION repmgr" to load this file. \quit
 
+ALTER FUNCTION set_repmgrd_pid(INT, TEXT) RETURNS NULL ON NULL INPUT;
+

repmgr--4.5.sql

@@ -176,7 +176,7 @@ CREATE FUNCTION get_repmgrd_pidfile()
 CREATE FUNCTION set_repmgrd_pid(INT, TEXT)
   RETURNS VOID
   AS 'MODULE_PATHNAME', 'set_repmgrd_pid'
-  LANGUAGE C STRICT;
+  LANGUAGE C CALLED ON NULL INPUT;
 
 CREATE FUNCTION repmgrd_is_running()
   RETURNS BOOL