lang/funcs: don't panic when transpose produces empty result

lang/funcs/collection.go

@@ -1179,7 +1179,6 @@ func sliceIndexes(args []cty.Value) (int, int, bool, error) {
 	return startIndex, endIndex, startKnown && endKnown, nil
 }
 
-// TransposeFunc contructs a function that takes a map of lists of strings and
 // TransposeFunc constructs a function that takes a map of lists of strings and
 // swaps the keys and values to produce a new map of lists of strings.
 var TransposeFunc = function.New(&function.Spec{
@@ -1226,6 +1225,10 @@ var TransposeFunc = function.New(&function.Spec{
 			outputMap[outKey] = cty.ListVal(values)
 		}
 
+		if len(outputMap) == 0 {
+			return cty.MapValEmpty(cty.List(cty.String)), nil
+		}
+
 		return cty.MapVal(outputMap), nil
 	},
 })

lang/funcs/collection_test.go

@@ -2903,8 +2903,8 @@ func TestTranspose(t *testing.T) {
 			cty.MapVal(map[string]cty.Value{
 				"key1": cty.ListValEmpty(cty.String),
 			}),
-			cty.NilVal,
-			true,
+			cty.MapValEmpty(cty.List(cty.String)),
+			false,
 		},
 		{ // bad map - value not a list
 			cty.MapVal(map[string]cty.Value{