[WifiTrackerLib] Fix NPE when refreshing link properties

Add a null check when referencing link properties due to a race
condition where a connected WifiEntry receives a link properties changed
callback that is null.

Bug: 148752497
Test: atest WifiTrackerLibTests
Change-Id: I86d6d1c48b2945e111f66122bd9ace91dd63db12
1 file changed