Fix PHP 8.1 "strlen(null)" exceptions in HarbormasterLintPropertyView

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.

```
HarbormasterLintPropertyView.php:42: strlen(): Passing null to parameter #1 ($string) of type string is deprecated
HarbormasterLintPropertyView.php:49: strlen(): Passing null to parameter #1 ($string) of type string is deprecated
```

Refs T16468

Test Plan: None.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16468

Differential Revision: https://we.phorge.it/D26702

src/applications/harbormaster/view/HarbormasterLintPropertyView.php

···
       $line = $message->getLine();
 
       $href = null;
-      if (strlen(idx($this->pathURIMap, $path))) {
+      if (phutil_nonempty_string(idx($this->pathURIMap, $path))) {
         $href = $this->pathURIMap[$path].max($line, 1);
       }
 
       $severity = $this->renderSeverity($message->getSeverity());
 
       $location = $path.':'.$line;
-      if (strlen($href)) {
+      if (phutil_nonempty_string($href)) {
         $location = phutil_tag(
           'a',
           array(