Fix the issue with empty array replacement

Prior to this change, if there is only one element in an array that is
a reference to an empty array, the original array becomes nil after
replacement, but it should be an empty array instead of nil.

Fixes tektoncd#5149

pkg/apis/pipeline/v1beta1/param_types.go

@@ -199,7 +199,7 @@ func (arrayOrString *ArrayOrString) ApplyReplacements(stringReplacements map[str
 	if arrayOrString.Type == ParamTypeString {
 		arrayOrString.StringVal = substitution.ApplyReplacements(arrayOrString.StringVal, stringReplacements)
 	} else {
-		var newArrayVal []string
+		newArrayVal := []string{}
 		for _, v := range arrayOrString.ArrayVal {
 			newArrayVal = append(newArrayVal, substitution.ApplyArrayReplacements(v, stringReplacements, arrayReplacements)...)
 		}

pkg/apis/pipeline/v1beta1/param_types_test.go

@@ -200,7 +200,14 @@ func TestArrayOrString_ApplyReplacements(t *testing.T) {
 		},
 		expectedOutput: v1beta1.NewArrayOrString("firstvalue", "array", "value", "lastvalue", "asdf", "sdfsd"),
 	}, {
-		name: "empty array replacement",
+		name: "empty array replacement without extra elements",
+		args: args{
+			input:             v1beta1.NewArrayOrString("$(arraykey)"),
+			arrayReplacements: map[string][]string{"arraykey": {}},
+		},
+		expectedOutput: &v1beta1.ArrayOrString{Type: v1beta1.ParamTypeArray, ArrayVal: []string{}},
+	}, {
+		name: "empty array replacement with extra elements",
 		args: args{
 			input:              v1beta1.NewArrayOrString("firstvalue", "$(arraykey)", "lastvalue"),
 			stringReplacements: map[string]string{"some": "value", "anotherkey": "value"},