tests: Bluetooth: tester: Fix missing net_buf_unref

This fixes missing unreference of network buffer allocated
when there is no data to send in the ring buffer.
The code have been refactored, so that we check whether there is
anything to send first, and if there is, the TX buffer is allocated.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>