gtbs: Fix crash when setting null uri

01-14 16:33:18.500 E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
01-14 16:33:18.500 E AndroidRuntime: 	at com.android.bluetooth.tbs.TbsGatt.setIncomingCall(TbsGatt.java:579)
01-14 16:33:18.500 E AndroidRuntime: 	at com.android.bluetooth.tbs.TbsGeneric.callAdded(TbsGeneric.java:421)

Bug: 214637136
Test: compile
Change-Id: Icae1520d68ec33d026dff3bd518a7b919a8b84d3