Check string variable against both empty and null string

The new RIL in O changes all null strings to empty strings, but the code
only checks some string variable against null in some areas of frameworks/
opt/telephony. This CL adds the check against both empty and null for
these strings.

Test: TreeHugger
Bug: 62490549
Change-Id: If18154e896666eb9d5398181b9efe7cce40d7f75
Merged-In: If18154e896666eb9d5398181b9efe7cce40d7f75
(cherry picked from commit 0b23fe58)