[Bug]: Formula returns 0.0 instead of null for string type

### Apache Hop version?

2.12

### Java version?

17

### Operating system

Windows

### What happened?

I recently discovered a weird behaviour in the formula transform.
If you look at the image, given the formula, NEW_FIELD should be null instead of "0.0".
If I select just one of the IDs though, then NEW_FIELD is correctly assigned null.

I have the same behaviour on both Hop 2.8 on Java 11 and Hop 2.12 on Java 17.

Thanks!

### Issue Priority

Priority: 2

### Issue Component

Component: Transforms

Start with the formula transform and reproduce the reported case on the listed Hop and Java versions, using the screenshots as the expected behavior reference. Investigate why the formula produces 0.0 when multiple IDs are selected but null when one ID is selected. Done means the affected expression returns null consistently for the string-type case.

Written by the indexing model from the issue text.