Fix PHP 8.1 "strlen(null)" exception adding a "Drydock: Run Command" build step in Harbormaster

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=customOAuthUrlencodeNull, ref.master=788098096e11, ref.customOAuthUrlencodeNull=4f0f2043b7e9), phorge(head=master, ref.master=226f3150933d)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/harbormaster/controller/HarbormasterStepEditController.php:142]
```

Closes T15592

Test Plan: Add a build step for an existing Harbormaster Build Plan, see that page `New Step: Drydock: Run Command` is correctly rendered at `/harbormaster/step/new/1/HarbormasterDrydockCommandBuildStepImplementation/`

Reviewers: O1 Blessed Committers, Matthew

Reviewed By: O1 Blessed Committers, Matthew

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

Maniphest Tasks: T15592

Differential Revision: https://we.phorge.it/D25377

src/applications/harbormaster/controller/HarbormasterStepEditController.php

@@ -139,7 +139,7 @@ final class HarbormasterStepEditController
       ->setUser($viewer);
 
     $instructions = $implementation->getEditInstructions();
-    if (strlen($instructions)) {
+    if (phutil_nonempty_string($instructions)) {
       $form->appendRemarkupInstructions($instructions);
     }