Fix navigation assertions that failed in conjunction with navigation validation tools (#858)

Fixes two assertions that were triggered by navigation validation tools:
- if the step stracer is set to record every step, make sure the barcode is only accessed when actually on surface
- since rays return zero for 'qop', make sure the free track parameter momentum is only written to file if it can be
  retrieved. Otherwise write the (normalized) direction to file

Author: niermann999

io/include/detray/io/csv/track_parameters.hpp

@@ -134,7 +134,9 @@ inline void write_free_track_params(
         for (const auto &[charge, track_param] : track_params) {
 
             const auto &glob_pos = track_param.pos();
-            const auto &p = track_param.mom(charge);
+            // Momentum may not be retrievable for straight-line tracks
+            const auto &p{charge != 0.f ? track_param.mom(charge)
+                                        : track_param.dir()};
 
             io::csv::free_track_parameters track_param_data{};
             track_param_data.track_id = track_idx;

tests/include/detray/test/validation/step_tracer.hpp

@@ -95,9 +95,16 @@ struct step_tracer : actor {
         // Collect the data whenever requested
         if (navigation.is_on_surface() || tracer_state.m_collect_every_step) {
 
-            step_data_t sd{stepping.step_size(),         stepping.path_length(),
-                           stepping.n_total_trials(),    navigation.direction(),
-                           navigation.barcode(),         stepping(),
+            const geometry::barcode bcd{navigation.is_on_surface()
+                                            ? navigation.barcode()
+                                            : geometry::barcode{}};
+
+            step_data_t sd{stepping.step_size(),
+                           stepping.path_length(),
+                           stepping.n_total_trials(),
+                           navigation.direction(),
+                           bcd,
+                           stepping(),
                            stepping.transport_jacobian()};
 
             tracer_state.m_steps.push_back(std::move(sd));