AK+Everywhere: Remove the null state of DeprecatedString

This commit removes DeprecatedString's "null" state, and replaces all
its users with one of the following:
- A normal, empty DeprecatedString
- Optional<DeprecatedString>

Note that null states of DeprecatedFlyString/StringView/etc are *not*
affected by this commit. However, DeprecatedString::empty() is now
considered equal to a null StringView.

Userland/Libraries/LibPDF/Parser.cpp

@@ -256,8 +256,6 @@ NonnullRefPtr<StringObject> Parser::parse_string()
         is_binary_string = true;
     }
 
-    VERIFY(!string.is_null());
-
     auto string_object = make_object<StringObject>(string, is_binary_string);
 
     if (m_document->security_handler() && m_enable_encryption)