stamper: preserve empty array when stamping (#993)

when performing the recursive evaluation of the objects we stamp when
using `spec.template: {...}`, we used to always have a nil var that
would be instantiated when appending to it, but given that with 0-sized
slices we wouldn't get into the loop and thus never initialize, we'd
effectively always be transformating `[]` into `nil`.

`[]` -> `nil` is problematic specially when using `Runnable` with Tekton
as parameters that should be of type `array` but are initialized with
`[]` from the Runnable side would end up being transformed into `nil`
which is badly handled by tekton (it transforms `nil` -> `""`, which
goes against the validation of a task that expects an array)

Signed-off-by: Ciro S. Costa <[email protected]>

pkg/templates/stamper.go

@@ -120,6 +120,10 @@ func (s *Stamper) recursivelyEvaluateTemplates(jsonValue interface{}, loopDetect
 		}
 		return stampedMap, nil
 	case []interface{}:
+		if len(typedJSONValue) == 0 {
+			return typedJSONValue, nil
+		}
+
 		var stampedSlice []interface{}
 		for _, sliceElement := range typedJSONValue {
 			stampedElement, err := s.recursivelyEvaluateTemplates(sliceElement, loopDetector)

pkg/templates/stamper_test.go

@@ -210,6 +210,9 @@ var _ = Describe("Stamper", func() {
 			Entry(`Single tag, array value and type preserved, nested tags evaluated`,
 				`$(params.sub)$`, `["foo", "$(params['extra-for-nested'])$"]`, []interface{}{"foo", "nested"}, ""),
 
+			Entry(`Single tag, empty array value preserved`,
+				`$(params.sub)$`, `["foo", []]`, []interface{}{"foo", []interface{}{}}, ""),
+
 			Entry(`Multiple tags, result becomes a string`,
 				`$(params.sub)$$(params.sub)$`, `5`, "55", ""),