fix(NavigationManager): do not warn when an unknown entry is requested

get() is declared to return ?array and all of its callers handle null -
the public page layout explicitly renders an empty app list when the
current app has no navigation entry. The lookup itself did not guard
against a missing key though, so every public page rendered by an app
that registers no navigation entry (typical for files integration apps
like drawio) logged a PHP warning per page view:

    Undefined array key "drawio" at lib/private/NavigationManager.php#432

Returning null explicitly keeps the behaviour identical for every
caller - the expression already evaluated to null after the warning -
and removes the log noise.

Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

Author: solracsf

lib/private/NavigationManager.php

@@ -312,7 +312,7 @@ public function setUnreadCounter(string $id, int $unreadCounter): void {
 	#[Override]
 	public function get(string $id): ?array {
 		$this->resolveAppNavigationEntries();
-		return $this->entries[$id];
+		return $this->entries[$id] ?? null;
 	}
 
 	#[Override]

tests/lib/NavigationManagerTest.php

@@ -171,6 +171,13 @@ public function testAddClosure(array $entry, array $expectedEntry): void {
 		$this->assertEmpty($this->navigationManager->getAll('all'), 'Expected no navigation entry exists after clear()');
 	}
 
+	public function testGetUnknownEntryReturnsNull(): void {
+		// Requesting an entry no app has registered must return null without
+		// emitting an "Undefined array key" warning, e.g. when an app without
+		// a navigation entry renders a public page
+		$this->assertNull($this->navigationManager->get('unknown'));
+	}
+
 	public function testAddArrayClearGetAll(): void {
 		$entry = [
 			'id' => 'entry id',