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/Applications/SoundPlayer/Player.cpp

@@ -40,9 +40,6 @@ Player::Player(Audio::ConnectionToServer& audio_client_connection)
 
 void Player::play_file_path(DeprecatedString const& path)
 {
-    if (path.is_null())
-        return;
-
     if (!FileSystem::exists(path)) {
         audio_load_error(path, "File does not exist"sv);
         return;