Fix "GATT Read Multiple Variable Response" builder

0 length value is perfectly fine, and should result in just length
added into the packet.
Currently, for 0 length value we just break out of loop, and don't add
any value.
This means, that if first characetristic in response had 0 length, we
would return empty packet.

Bug: 352696105
Flag: exempt, obvious logic fix
Change-Id: Ida4f6b566cf9fa40fc5330d8084c29669ccaa608