Do not inform user when the alpha identifier is a null data

Add a condition to distinguish between alpha identifier with
null data and no alpha identifier.

Ref spec: 3GPP TS 11.14, 6.4.5. Play Tone.

Bug: 22980070
Test: manual
Change-Id: Ibfe211653c34206e4326d4b4f234c9dd1e9778f2