[ci] remove default value for target branch

The idea is to evaluate differences between the current branch and the 
target branch for merge requests. Having a default we don't notice if 
this breaks when someone removes the target branch variable passed from 
the parent pipeline.

.gitlab-ci/default.yml

@@ -10,9 +10,10 @@ workflow:
   rules:
     - if: $CI_PIPELINE_SOURCE=="parent_pipeline"
 
+# variables that may be overwritten by the trigger
 variables:
-  TRIGGER_SOURCE: "unknown"
-  MR_TARGET_BRANCH_NAME: "master"
+  TRIGGER_SOURCE: "undefined"
+  MR_TARGET_BRANCH_NAME: "undefined"
 
 select tests:
   stage: configure
@@ -20,7 +21,7 @@ select tests:
     - |
       dunecontrol --opts=$DUNE_OPTS_FILE --current all
       if [[ "$TRIGGER_SOURCE" == "merge_request_event" ]]; then
-          echo "Detecting changes w.r.t to branch $MR_TARGET_BRANCH_NAME"
+          echo "Detecting changes w.r.t to target branch '$MR_TARGET_BRANCH_NAME'"
           python3 bin/testing/getchangedfiles.py -o changedfiles.txt -t origin/$MR_TARGET_BRANCH_NAME
           python3 bin/testing/findtests.py -o affectedtests.json --file-list changedfiles.txt --build-dir build-cmake
       else