Remove assertion from PortalRunMulti

We have an assertion to ensure that a command tag has been assigned by
the time we're done executing, but if we happen to execute a command
with no queries, the assertion would fail.  Per discussion, rather than
contort things to get a tag assigned, just remove the assertion.

Oversight in 2f9661311b.  That commit also retained a comment that
explained logic that had been adjacent to it but diffused into various
places, leaving none apt to keep part of the comment.  Remove that part,
and rewrite what remains for extra clarity.

Bug: #18984
Backpatch-through: 13
Reported-by: Aleksander Alekseev <aleksander@tigerdata.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Michaël Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/18984-0f4778a6599ac3ae@postgresql.org

src/backend/tcop/pquery.c

@@ -1353,24 +1353,15 @@ PortalRunMulti(Portal portal,
 		PopActiveSnapshot();
 
 	/*
-	 * If a query completion data was supplied, use it.  Otherwise use the
-	 * portal's query completion data.
-	 *
-	 * Exception: Clients expect INSERT/UPDATE/DELETE tags to have counts, so
-	 * fake them with zeros.  This can happen with DO INSTEAD rules if there
-	 * is no replacement query of the same type as the original.  We print "0
-	 * 0" here because technically there is no query of the matching tag type,
-	 * and printing a non-zero count for a different query type seems wrong,
-	 * e.g.  an INSERT that does an UPDATE instead should not print "0 1" if
-	 * one row was updated.  See QueryRewrite(), step 3, for details.
+	 * If a command tag was requested and we did not fill in a run-time-
+	 * determined tag above, copy the parse-time tag from the Portal.  (There
+	 * might not be any tag there either, in edge cases such as empty prepared
+	 * statements.  That's OK.)
 	 */
-	if (qc && qc->commandTag == CMDTAG_UNKNOWN)
-	{
-		if (portal->qc.commandTag != CMDTAG_UNKNOWN)
-			CopyQueryCompletion(qc, &portal->qc);
-		/* If the caller supplied a qc, we should have set it by now. */
-		Assert(qc->commandTag != CMDTAG_UNKNOWN);
-	}
+	if (qc &&
+		qc->commandTag == CMDTAG_UNKNOWN &&
+		portal->qc.commandTag != CMDTAG_UNKNOWN)
+		CopyQueryCompletion(qc, &portal->qc);
 }
 
 /*