Avoid attempting to lock invalid shard identifier

A recent change generates a "dummy" shard placement with its identifier
set to INVALID_SHARD_ID for SELECT queries against distributed tables
with no shards. Normally, no lock is acquired for SELECT statements,
but if all_modifications_commutative is set to true, we will acquire a
shared lock, triggering an assertion failure within LockShardResource
in the above case.

The "dummy" shard placement is actually necessary to ensure such empty
queries have somewhere to execute, and INVALID_SHARD_ID seems the most
appropriate value for the dummy's shard identifier field, so the most
straightforward fix is to just avoid locking invalid shard identifiers.

src/backend/distributed/executor/multi_router_executor.c

@@ -294,7 +294,10 @@ AcquireExecutorShardLock(Task *task, LOCKMODE lockMode)
 {
 	int64 shardId = task->anchorShardId;
 
+	if (shardId != INVALID_SHARD_ID)
+	{
 		LockShardResource(shardId, lockMode);
+	}
 }