GetChildReferences checks for non-nil TaskRuns

In tektoncd#5006, we refactored
`GetChildReferences` such that it checks that `TaskRun` is non-nil
before calling the helper function `getChildRefForTaskRun` that
expects a non-nil `TaskRun`.

In this change, we add a check that each `TaskRun` from matrixed
`PipelineTask` is non-nil before calling `getChildRefForTaskRun`.

Related PR: tektoncd#5008

pkg/reconciler/pipelinerun/resources/pipelinerunstate.go

@@ -249,7 +249,9 @@ func (state PipelineRunState) GetChildReferences() []v1beta1.ChildStatusReferenc
 			childRefs = append(childRefs, rprt.getChildRefForTaskRun(rprt.TaskRun))
 		case len(rprt.TaskRuns) != 0:
 			for _, taskRun := range rprt.TaskRuns {
-				childRefs = append(childRefs, rprt.getChildRefForTaskRun(taskRun))
+				if taskRun != nil {
+					childRefs = append(childRefs, rprt.getChildRefForTaskRun(taskRun))
+				}
 			}
 		}
 	}

pkg/reconciler/pipelinerun/resources/pipelinerunstate_test.go

@@ -2565,6 +2565,34 @@ func TestPipelineRunState_GetChildReferences(t *testing.T) {
 				PipelineTaskName: "single-custom-task-1",
 			}},
 		},
+		{
+			name: "unresolved-matrixed-task",
+			state: PipelineRunState{{
+				TaskRunNames: []string{"task-run-0", "task-run-1", "task-run-2", "task-run-3"},
+				PipelineTask: &v1beta1.PipelineTask{
+					Name: "matrixed-task",
+					TaskRef: &v1beta1.TaskRef{
+						Name:       "task",
+						Kind:       "Task",
+						APIVersion: "v1beta1",
+					},
+					WhenExpressions: []v1beta1.WhenExpression{{
+						Input:    "foo",
+						Operator: selection.In,
+						Values:   []string{"foo", "bar"},
+					}},
+					Matrix: []v1beta1.Param{{
+						Name:  "foobar",
+						Value: v1beta1.ArrayOrString{Type: v1beta1.ParamTypeArray, ArrayVal: []string{"foo", "bar"}},
+					}, {
+						Name:  "quxbaz",
+						Value: v1beta1.ArrayOrString{Type: v1beta1.ParamTypeArray, ArrayVal: []string{"qux", "baz"}},
+					}},
+				},
+				TaskRuns: []*v1beta1.TaskRun{nil, nil, nil, nil},
+			}},
+			childRefs: nil,
+		},
 		{
 			name: "matrixed-task",
 			state: PipelineRunState{{