[Store] Add withContent validation test case

Add test case to verify that TextDocument.withContent() properly validates
the new content parameter and throws InvalidArgumentException when given
empty or whitespace-only content.

Author: OskarStark

src/store/tests/Document/TextDocumentTest.php

@@ -265,4 +265,15 @@ public function testWithContent()
         $this->assertSame($metadata, $updatedDocument->metadata);
         $this->assertSame($originalContent, $originalDocument->content);
     }
+
+    #[TestDox('withContent validates new content')]
+    public function testWithContentValidatesContent()
+    {
+        $document = new TextDocument(Uuid::v4(), 'Valid content');
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The content shall not be an empty string.');
+
+        $document->withContent('   ');
+    }
 }