Bluetooth: Prevent gatt DB copy if server cache is absent

In case the server cache does not exist for a specific
connection, the API used to copy the server DB into the
DB structure needs to return immediately. This change
makes sure of that and prevents dereferencing of a null
block causing the runtime error.

Change-Id: Iec3040a1280ef9d80b1b9c76eca8071dff499411