Allow null values in Compact function

The function would previously panic when one or more null values were among the arguments.

The new behavior treats nulls as empty strings, therefore, it removes them.

lang/funcs/collection.go

@@ -246,7 +246,7 @@ var CompactFunc = function.New(&function.Spec{
 
 		for it := listVal.ElementIterator(); it.Next(); {
 			_, v := it.Element()
-			if v.AsString() == "" {
+			if v.IsNull() || v.AsString() == "" {
 				continue
 			}
 			outputList = append(outputList, v)

lang/funcs/collection_test.go

@@ -572,6 +572,7 @@ func TestCompact(t *testing.T) {
 				cty.StringVal("test"),
 				cty.StringVal(""),
 				cty.StringVal("test"),
+				cty.NullVal(cty.String),
 			}),
 			cty.ListVal([]cty.Value{
 				cty.StringVal("test"),
@@ -588,6 +589,14 @@ func TestCompact(t *testing.T) {
 			cty.ListValEmpty(cty.String),
 			false,
 		},
+		{
+			cty.ListVal([]cty.Value{
+				cty.NullVal(cty.String),
+				cty.NullVal(cty.String),
+			}),
+			cty.ListValEmpty(cty.String),
+			false,
+		},
 		{
 			cty.ListValEmpty(cty.String),
 			cty.ListValEmpty(cty.String),
@@ -610,6 +619,7 @@ func TestCompact(t *testing.T) {
 				cty.StringVal("test"),
 				cty.UnknownVal(cty.String),
 				cty.StringVal(""),
+				cty.NullVal(cty.String),
 			}),
 			cty.UnknownVal(cty.List(cty.String)),
 			false,