fix: avoid panic when validate enum param with special matrix task

fix #8464

If the matrix Task has no TaskRun to execute, the `ResolvedTask` will be
nil, we should skip the validation.

pkg/reconciler/pipelinerun/resources/pipelinerunresolution.go

@@ -886,6 +886,10 @@ func createResultsCacheMatrixedTaskRuns(rpt *ResolvedPipelineTask) (resultsCache
 // ValidateParamEnumSubset finds the referenced pipeline-level params in the resolved pipelineTask.
 // It then validates if the referenced pipeline-level param enums are subsets of the resolved pipelineTask-level param enums
 func ValidateParamEnumSubset(pipelineTaskParams []v1.Param, pipelineParamSpecs []v1.ParamSpec, rt *resources.ResolvedTask) error {
+	// When the matrix Task has no TaskRun, the rt will be nil, we should skip the validation.
+	if rt == nil {
+		return nil
+	}
 	for _, p := range pipelineTaskParams {
 		// calculate referenced param enums
 		res, present, errString := substitution.ExtractVariablesFromString(p.Value.StringVal, "params")

pkg/reconciler/pipelinerun/resources/pipelinerunresolution_test.go

@@ -5446,6 +5446,29 @@ func TestValidateParamEnumSubset_Valid(t *testing.T) {
 					},
 				},
 			},
+		}, {
+			name: "rt is nil - pass",
+			params: []v1.Param{
+				{
+					Name: "resolved-task-p1",
+					Value: v1.ParamValue{
+						StringVal: "$(params.p1) and $(params.p2)",
+					},
+				},
+			},
+			pipelinePs: []v1.ParamSpec{
+				{
+					Name: "p1",
+					Type: v1.ParamTypeString,
+					Enum: []string{"v1", "v2"},
+				},
+				{
+					Name: "p2",
+					Type: v1.ParamTypeString,
+					Enum: []string{"v3", "v4"},
+				},
+			},
+			rt: nil,
 		},
 	}
 
@@ -5530,6 +5553,7 @@ func TestValidateParamEnumSubset_Invalid(t *testing.T) {
 				},
 			},
 		},
+		rt:      &resources.ResolvedTask{},
 		wantErr: errors.New("unexpected error in ExtractVariablesFromString: Invalid referencing of parameters in \"$(params.p1.aaa.bbb)\"! Only two dot-separated components after the prefix \"params\" are allowed."),
 	}}