Fix PHP 8.1 "strlen(null)" exception adding a Harbormaster Build Plan

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/editor/HarbormasterBuildPlanEditEngine.php:106]
```

Closes T15591

Test Plan: Visit `/harbormaster/plan/edit/form/default/`. "Create Build Plan" page renders as expected.

Reviewers: O1 Blessed Committers, Matthew

Reviewed By: O1 Blessed Committers, Matthew

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

Maniphest Tasks: T15591

Differential Revision: https://we.phorge.it/D25376