Remove default value for FailureContext artifacts parameter

Developers are not expected to instantiate FailureContext directly. The testing framework always instantiates FailureContext when invoking TestFailureHandlers and always passes the list of captured failure artifacts, making the default emptyList() argument unnecessary.

Fixes: 553092225
Test: FailurePipelineTest
Relnote: NA
Change-Id: I38779b1399c1c4428d006e364313e7197c764709