functions: Fix defaults null collections panic

When applying default values to collection types, null collections in
the input should result in empty collections in the output.

lang/funcs/defaults.go

@@ -127,9 +127,11 @@ func defaultsApply(input, fallback cty.Value) cty.Value {
 		case wantTy.IsMapType():
 			newVals := map[string]cty.Value{}
 
-			for it := input.ElementIterator(); it.Next(); {
-				k, v := it.Element()
-				newVals[k.AsString()] = defaultsApply(v, fallback)
+			if !input.IsNull() {
+				for it := input.ElementIterator(); it.Next(); {
+					k, v := it.Element()
+					newVals[k.AsString()] = defaultsApply(v, fallback)
+				}
 			}
 
 			if len(newVals) == 0 {
@@ -139,10 +141,12 @@ func defaultsApply(input, fallback cty.Value) cty.Value {
 		case wantTy.IsListType(), wantTy.IsSetType():
 			var newVals []cty.Value
 
-			for it := input.ElementIterator(); it.Next(); {
-				_, v := it.Element()
-				newV := defaultsApply(v, fallback)
-				newVals = append(newVals, newV)
+			if !input.IsNull() {
+				for it := input.ElementIterator(); it.Next(); {
+					_, v := it.Element()
+					newV := defaultsApply(v, fallback)
+					newVals = append(newVals, newV)
+				}
 			}
 
 			if len(newVals) == 0 {

lang/funcs/defaults_test.go

@@ -370,6 +370,25 @@ func TestDefaults(t *testing.T) {
 			Defaults: cty.StringVal("hello"),
 			WantErr:  `only object types and collections of object types can have defaults applied`,
 		},
+		// When applying default values to collection types, null collections in the
+		// input should result in empty collections in the output.
+		{
+			Input: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.NullVal(cty.List(cty.String)),
+				"b": cty.NullVal(cty.Map(cty.String)),
+				"c": cty.NullVal(cty.Set(cty.String)),
+			}),
+			Defaults: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("hello"),
+				"b": cty.StringVal("hi"),
+				"c": cty.StringVal("greetings"),
+			}),
+			Want: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.ListValEmpty(cty.String),
+				"b": cty.MapValEmpty(cty.String),
+				"c": cty.SetValEmpty(cty.String),
+			}),
+		},
 	}
 
 	for _, test := range tests {