Need to check empty not null for String

getWcdmaLevel needs to check empty string, which is currently doesn't
because mWcdmaDefaultSignalMeasurement could be not null but empty string.

Test: TelephonybaseTest

Change-Id: If00a5953eef972250266ceef234cce213d45bc6a
Signed-off-by: Seongho Kim <sh_tiger.kim@samsung.com>