[Bug]: JSON Output NULL Colums after BugFix #2839

### Apache Hop version?

2.5.0-SNAPSHOT

### Java version?

openjdk 11.0.19

### Operating system

Docker

### What happened?

If the result of the JSON Output is smaller then nrRowsInBloc in the other columns, the JSON Output returns NULL.
I think this is because the function processRow() is missing the call outputRow(prevRow) and the definition of prevRow.

### Issue Priority

Priority: 1

### Issue Component

Component: Transforms

Start with the JSON Output transform's processRow() implementation and inspect how outputRow(prevRow) and prevRow are handled when the result has fewer rows than nrRowsInBloc. Reproduce the issue on Apache Hop 2.5.0-SNAPSHOT with Java 11 in Docker, then verify that the other columns no longer become NULL for shorter JSON results.

Written by the indexing model from the issue text.