API fix: getProprietaryCallExtras should not return null

Bug: 149216142
Fixes: 151172594
Test: atest CtsTelephonyTestCases
Merged-In: I7bd0d44f08169d41396a8cb202f3928e14bc5939
Change-Id: I7bd0d44f08169d41396a8cb202f3928e14bc5939
3 files changed