Send empty string for redial

Test: 1. Enable HFP Client.
2. Discover, pair and connect to remote device.
3. Once connected, redial from hf client.

Failure:
There is a crash since NULL is sent from application
to the stack for redial.

Rootcause:
Application sending NULL for redial to the stack.

Fix:
For redial, NULL is sent from application to stack.
Due to NULL checks in JNI, there was an exception
generated while calling stack API. Send empty string
instead of null.

Fixes: 35415608
Change-Id: I1ee4b8c5fa43e005da228df1bd260d99d96a8913