templates: Handle missing values in issues and incidents

Handle some missing values in issue and incident templates properly.

The 'default' filter unfortunately handles 'undefined' only, so use a
combination of the filter and the tests to handle both interim and final
None values when accessing fields deep in a structure.

kcidb/templates/incident.j2

@@ -3,10 +3,13 @@
 {% macro summary(incident) %}
     {%- if incident.test -%}
         {{- "Incident in " +
-            (incident.test.path | default("an unknown test")) +
+            ("an unknown test"
+             if incident.test.path | default(none) is none
+             else incident.test.path) +
             " on " +
-            (incident.test.build.architecture |
-             default("an unknown architecture")) -}}
+            ("an unknown architecture"
+             if incident.test.build.architecture | default(none) is none
+             else incident.test.build.architecture) -}}
         {%- if incident.issue.report_subject -%}
             {{- ": " + incident.issue.report_subject -}}
         {%- endif -%}

kcidb/templates/issue.j2

@@ -42,7 +42,9 @@
     In {{ branches | length -}}
     {{- " branch" if (branches | length) == 1 else " branches" }}:
         {% for branch in branches[:max_list_len] %}
-            {{- "        " + branch[0] + " " + branch[1] }}
+            {{- "        " +
+                ("?" if branch[0] is none else branch[0]) + " " +
+                ("?" if branch[1] is none else branch[1]) }}
         {% endfor %}
         {% if (branches | length) > max_list_len %}
             {{- "        ..." }}