core: Fix command line arguments written by DesMetrics

The DesMetrics::Initialize branch conditions were swapped, so the command_line_arguments field of the JSON trace header recorded the [argv empty or not available] placeholder when arguments were present, and recorded nothing when they were absent.

Fix the swapped branches and add a des-metrics unit test suite that verifies both cases (arguments present, and empty).

This bug is only observable when DES metrics is enabled at configure time (--enable-des-metrics), but the fix is a trivial branch swap and the test exercises DesMetrics directly, so it runs in the standard test matrix.