Add a null check to prevent NullPointerException

TelephonyManager#getAllCellInfo() may return null, so before
running the foreach loop, the null check is needed.

Bug: 156567547
Test: atest NetworkStackTests:NetworkMonitorTest
Change-Id: I925eefaffeadd87ef1b18204cbf82a9e63779a10
Merged-In: I5d5e98add0ea2abf92156c11188739c535e7e9a8
(cherry picked from commit 9d5cc530)