gtbs: Fix for crash when uri is not set

java.lang.NullPointerException: Attempt to invoke virtual method 'byte[] java.lang.String.getBytes()' on a null object reference
	at com.android.bluetooth.tbs.TbsGatt.setBearerListCurrentCalls(TbsGatt.java:496)
	at com.android.bluetooth.tbs.TbsGeneric.notifyCclc(TbsGeneric.java:885)
	at com.android.bluetooth.tbs.TbsGeneric.currentCallsList(TbsGeneric.java:549)

Bug: 215625928
Test: compile
Change-Id: I42ff9274bf29b1f5ca2b118375e37fa1d174e4b2