Work around exception in Herald transcript of old tasks: Call to a member function getAppliedTransactionPHIDs() on bool

Summary:
In some cases, older Herald transcripts which got garbage collected display an exception.
For unknown reasons, calling `getObjectTranscript()` on the HeraldTranscript object `$xscript` can return a bool with the value `false`.
Afterwards, calling `getAppliedTransactionPHIDs()` on that bool throws an exception.
Thus as a workaround, return an empty array instead in this case, like the code already does when $xscript->getObjectTranscript()->getAppliedTransactionPHIDs() === null.

Note that it remains unclear whether/when the bool may have the value `true` hence this situation is not covered by this code change (the code would still throw an exception in this situation).

Closes T15343

Test Plan:
1. Change system time to one year back (or optionally, use `./phorge/bin/garbage set-policy --collector herald.transcripts --days 1`, drop steps 9 and 10, and wait a day before performing step 11)
2. As an admin, go to http://phorge.localhost/herald/create/
3. Select "Maniphest Tasks"
4. On http://phorge.localhost/herald/create/?adapter=HeraldManiphestTaskAdapter , select "Global Rule"
5. Under "Conditions", select `any of` and set the three conditions `Description | contains | Internet Archive`, `Description | contains | archive.org`, `Description | contains | Wayback Machine`.
6. Under "Action", select `Add projects | someProject`
7. Select "Save Rule"
8. Trigger the rule by creating a new task.
9. Change system time back to today
10. On the shell, run `./phorge/bin/phd restart`
11. Visit created task and select "View Herald Transcript"
12. Transcript page displays "Old Transcript - Details of this transcript have been garbage collected." and shows no exception anymore

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15343

Differential Revision: https://we.phorge.it/D25246

src/applications/herald/controller/HeraldTranscriptController.php

@@ -717,8 +717,8 @@ final class HeraldTranscriptController extends HeraldController {
       ->setName(pht('Field Values'))
       ->setIcon('fa-file-text-o');
 
-    $xaction_phids = $this->getTranscriptTransactionPHIDs($xscript);
+    $has_xactions = $xscript->getObjectTranscript()
-    $has_xactions = (bool)$xaction_phids;
+      && $this->getTranscriptTransactionPHIDs($xscript);
 
     $nav->newLink('xactions')
       ->setName(pht('Transactions'))