Add null check to Task#equals()

Adds a couple base conditions to Task#equals -- essentially so that if the comparison target is null, returns false.

In Java Platform SE 8 docs, Object#equals() implements "For any non-null reference value x, x.equals(null) should return false. Since Task#equals() overrides Object.equals(), we should implement this too.

Bug: 274835596
Test: Manual
Change-Id: If8e23f620c4f2c88d1e058325fdebc54022acf3b