Fixed nil-pointer panic in strs:join

ucl/builtins/lists_test.go

@@ -18,6 +18,7 @@ func TestLists_Add(t *testing.T) {
 		{desc: "list add 1", eval: `lists:add [1 2 3] 4`, want: []any{1, 2, 3, 4}},
 		{desc: "list add 2", eval: `lists:add [1 2 3] 4 5`, want: []any{1, 2, 3, 4, 5}},
 		{desc: "list add 3", eval: `lists:add [1 2 3]`, want: []any{1, 2, 3}},
+		{desc: "list add 4", eval: `lists:add [1 2 3] () 4 () 5`, want: []any{1, 2, 3, nil, 4, nil, 5}},
 	}
 
 	for _, tt := range tests {

ucl/builtins/strs.go

@@ -67,7 +67,9 @@ func join(ctx context.Context, args ucl.CallArgs) (any, error) {
 		if i > 0 {
 			sb.WriteString(joinStr)
 		}
-		sb.WriteString(l.Index(i).String())
+		if o := l.Index(i); o != nil {
+			sb.WriteString(o.String())
+		}
 	}
 
 	return sb.String(), nil

ucl/builtins/strs_test.go

@@ -140,6 +140,7 @@ func TestStrs_Join(t *testing.T) {
 		{desc: "join 2", eval: `strs:join ["a" "b" "c"] "\n"`, want: "a\nb\nc"},
 		{desc: "join 3", eval: `strs:join ["a" "b" "c"] ""`, want: "abc"},
 		{desc: "join 4", eval: `strs:join [] ","`, want: ""},
+		{desc: "join 5", eval: `strs:join [() () () ()] ","`, want: ",,,"},
 
 		// Hmm, not super happy about this one...
 		{desc: "join 5", eval: `strs:join ["a" "b"] ["what"]`, want: "a[what]b"},