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/LibDNS/Packet.cpp

@@ -161,7 +161,7 @@ Optional<Packet> Packet::from_raw_packet(u8 const* raw_data, size_t raw_size)
         case RecordType::AAAA:
             // Fall through
         case RecordType::SRV:
-            data = { record.data(), record.data_length() };
+            data = ReadonlyBytes { record.data(), record.data_length() };
             break;
         default:
             // FIXME: Parse some other record types perhaps?