Fix "TypeError: Cannot read property 'x' of null" on SF traces

The protobuf definition allows the position attribute not to be defined.
Winscope crashes if the position is not defined, instead of assuming it
as (x: 0, y: 0)

Test: Load a SF protobuf object without the position attribute. Check if
Winscope does't crash while loading it.

Change-Id: Ib12ba7875f1e7b22819274c1cd9961a1dbfe82cf
1 file changed