Minor improvement about the tool document and the error msg

The patch includes
1. Add the document for the ase_handle assignment feature
2. Print "The iso data is empty" as the iso sdu data length is zero, it
could be useful to debug the audio issue

Bug: 204320094
Bug: 150670922
Test: dump ISO data with btsnoop contains the iso sdu data empty issue
Change-Id: Ic0857d0e21a568ecd52c98a15f5820a8161d7a14