Support SF traces without any layers (needed when booting SF)

When using continuous logging and having the trace enabled by default,
the first entries have no layers and crash Winscope.

Test: Flash a device with SF tracing enabled by default. Record the
initial trace. Open trace in Winscope.

Change-Id: I693815ff3e6468291770d3e8197e21bf75a2a87a
1 file changed