logical replication of truncate command with trigger causes Assert

Hackers,

On master, when a statement level trigger is fired for a replicated truncate command, the following stack trace is generated:

TRAP: FailedAssertion("portal != NULL", File: "pquery.c", Line: 1760, PID: 93854)
0   postgres                            0x0000000108e269f2 ExceptionalCondition + 130
1   postgres                            0x0000000108bef2f4 EnsurePortalSnapshotExists + 100
2   postgres                            0x0000000108a93231 _SPI_execute_plan + 529
3   postgres                            0x0000000108a93c0f SPI_execute_plan_with_paramlist + 127
4   plpgsql.so                          0x00000001098bf9e5 exec_stmt_execsql + 277
5   plpgsql.so                          0x00000001098bbaf6 exec_stmts + 294
6   plpgsql.so                          0x00000001098bb367 exec_stmt_block + 1127
7   plpgsql.so                          0x00000001098ba57a plpgsql_exec_trigger + 442
8   plpgsql.so                          0x00000001098cb5b1 plpgsql_call_handler + 305
9   postgres                            0x0000000108a3137c ExecCallTriggerFunc + 348
10  postgres                            0x0000000108a3447d afterTriggerInvokeEvents + 1517
11  postgres                            0x0000000108a33bb0 AfterTriggerEndQuery + 128
12  postgres                            0x0000000108a1a9e2 ExecuteTruncateGuts + 2210
13  postgres                            0x0000000108b83369 apply_dispatch + 3913
14  postgres                            0x0000000108b82185 LogicalRepApplyLoop + 485
15  postgres                            0x0000000108b81f87 ApplyWorkerMain + 1047
16  postgres                            0x0000000108b474a2 StartBackgroundWorker + 386
17  postgres                            0x0000000108b55cf6 maybe_start_bgworkers + 1254
18  postgres                            0x0000000108b54510 sigusr1_handler + 464
19  libsystem_platform.dylib            0x00007fff69f3d5fd _sigtramp + 29
20  ???                                 0x0000000000000000 0x0 + 0
21  postgres                            0x0000000108b537ae PostmasterMain + 3726
22  postgres                            0x0000000108aaa140 help + 0
23  libdyld.dylib                       0x00007fff69d44cc9 start + 1
24  ???                                 0x0000000000000004 0x0 + 4

I believe the issue was introduced in commit 84f5c2908da which added EnsurePortalSnapshotExists.   That's not going to work in the case of logical replication, because there isn't an ActivePortal nor a snapshot.

Attached patch v1-0001 reliably reproduces the problem, though you have to Ctrl-C out of it, because the logical replication gets stuck in a loop after the Assert is triggered.  You can see the stack trace by opening tmp_check/log/021_truncate_subscriber.log