Require a base job

This makes base jobs required and allows for a per-tenant default.

Story: 2001110
Task: 4793
Change-Id: I26ffddad8358c156cfac749ce98af70f3447f671
diff --git a/tests/fixtures/layouts/merging-github.yaml b/tests/fixtures/layouts/merging-github.yaml
index 9f43f75..c9673b9 100644
--- a/tests/fixtures/layouts/merging-github.yaml
+++ b/tests/fixtures/layouts/merging-github.yaml
@@ -13,6 +13,10 @@
         merge: true
         comment: false
 
+- job:
+    name: base
+    parent: null
+
 - project:
     name: org/project
     merge: