Check 'number' against both empty and null string for handleCdmaCallWaiting

The new RIL in O changes all null strings to empty strings, but the code
only checks 'number' against null for handleCdmaCallWaiting. This CL adds
the check against both empty and null strings.

Test: TreeHugger
Bug: 62490549
Change-Id: I9f4f577c40bcd5caa2a9bb1fd4c9e9c4c27e318c
Merged-In: I9f4f577c40bcd5caa2a9bb1fd4c9e9c4c27e318c
1 file changed