Fix invalid value of pg_aios.pid, function pg_get_aios()

When the value of pg_aios.pid is found to be 0, the function had the
idea to set "nulls" to "false" instead of "true", without setting the
value stored in the tuplestore.  This could lead to the display of buggy
data.  The intention of the code is clearly to display NULL when a PID
of 0 is found, and this commit adjusts the logic to do so.

Issue introduced by 60f566b4f2.

Author: ChangAo Chen <cca5507@qq.com>
Reviewed-by:  Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/tencent_7D61A85D6143AD57CA8D8C00DEC541869D06@qq.com
Backpatch-through: 18

src/backend/storage/aio/aio_funcs.c

@@ -149,7 +149,7 @@ retry:
 		if (owner_pid != 0)
 			values[0] = Int32GetDatum(owner_pid);
 		else
-			nulls[0] = false;
+			nulls[0] = true;
 
 		/* column: IO's id */
 		values[1] = Int32GetDatum(ioh_id);