WorkSource: Fix corner-case in equals method.

We should treat WorkSource objects that have a null mChains List
as being equal to one that has an empty non-null mChains List.

This bug was uncovered by the tests added to WifiController in
commit 0068bf953ad25e in frameworks/opt/net/wifi.

Bug: 62390666
Test: WorkSourceTest
Change-Id: Icd1f86aa5178a7beb436621362cfe771a3440398