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/LibWeb/XHR/XMLHttpRequest.cpp

@@ -512,7 +512,7 @@ WebIDL::ExceptionOr<void> XMLHttpRequest::open(String const& method_string, Stri
     // Unset this’s upload listener flag.
     m_upload_listener = false;
     // Set this’s request method to method.
-    m_request_method = move(normalized_method);
+    m_request_method = normalized_method.span();
     // Set this’s request URL to parsedURL.
     m_request_url = parsed_url;
     // Set this’s synchronous flag if async is false; otherwise unset this’s synchronous flag.