fix: cannot parse just a token

pkg/tokenizer/tokenizer.go

@@ -55,7 +55,11 @@ func getToken(i *iterator.Iterator[rune]) (*Token, error) {
 	index := i.Index()
 	for {
 		if i.IsDone() {
-			return nil, nil
+			return &Token{
+				Index: index,
+				Type:  TokenVariable,
+				Value: atom,
+			}, nil
 		}
 
 		pop, err := i.Peek()