LibPDF: Allow pages with no specified contents

The contents object may be omitted as per spec, which will just leave
the page blank.

Userland/Libraries/LibPDF/Document.cpp

@@ -116,7 +116,9 @@ PDFErrorOr<Page> Document::get_page(u32 index)
     else
         resources = adopt_ref(*new DictObject({}));
 
-    auto contents = TRY(raw_page_object->get_object(this, CommonNames::Contents));
+    RefPtr<Object> contents;
+    if (raw_page_object->contains(CommonNames::Contents))
+        contents = TRY(raw_page_object->get_object(this, CommonNames::Contents));
 
     Rectangle media_box;
     auto maybe_media_box_object = TRY(get_inheritable_object(CommonNames::MediaBox, raw_page_object));

Userland/Libraries/LibPDF/Document.h

@@ -30,7 +30,7 @@ struct Rectangle {
 
 struct Page {
     NonnullRefPtr<DictObject> resources;
-    NonnullRefPtr<Object> contents;
+    RefPtr<Object> contents;
     Rectangle media_box;
     Rectangle crop_box;
     float user_unit;

Userland/Libraries/LibPDF/Renderer.cpp

@@ -85,6 +85,9 @@ Renderer::Renderer(RefPtr<Document> document, Page const& page, RefPtr<Gfx::Bitm
 
 PDFErrorsOr<void> Renderer::render()
 {
+    if (m_page.contents.is_null())
+        return {};
+
     // Use our own vector, as the /Content can be an array with multiple
     // streams which gets concatenated
     // FIXME: Text operators are supposed to only have effects on the current