Add test for Flags.data_flow_trace

`CollectDataFlow()` uses `Flags.collect_data_flow` and
`Flags.data_flow_trace` at the same time. But in the null check before
the invocation, only `Flags.collect_data_flow` is checked, and there is
no other method to make sure `Flags.data_flow_trace` is not null, so
adding a null check for `Flags.data_flow_trace`.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D145040