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.

Merged-In: I693815ff3e6468291770d3e8197e21bf75a2a87a
Change-Id: I693815ff3e6468291770d3e8197e21bf75a2a87a
1 file changed