Fixed issue thrown upon empty markdown content save

Closes #2741

tests/Entity/PageTest.php

@@ -161,4 +161,27 @@ class PageTest extends TestCase
             'book_id' => $newBook->id,
         ]);
     }
+
+    public function test_empty_markdown_still_saves_without_error()
+    {
+        $this->setSettings(['app-editor' => 'markdown']);
+        $book = Book::query()->first();
+
+        $this->asEditor()->get($book->getUrl('/create-page'));
+        $draft = Page::query()->where('book_id', '=', $book->id)
+            ->where('draft', '=', true)->first();
+
+        $details = [
+            'name' => 'my page',
+            'markdown' => '',
+        ];
+        $resp = $this->post($book->getUrl("/draft/{$draft->id}"), $details);
+        $resp->assertRedirect();
+
+        $this->assertDatabaseHas('pages', [
+            'markdown' => $details['markdown'],
+            'id' => $draft->id,
+            'draft' => false
+        ]);
+    }
 }