Fix parsing of WM trace in critical mode

When tracing WM in critical mode, the children object is not always
assigned and this previously crashed the WM trace parsing in winscope.

Fix the WM trace parsing by assuming all null children as empty arrays

Test: run winscope, collect critical trace, open critical trace in winscope
Change-Id: I108c0725f3e9a27f45fda790d411614e0ab988bf