Improves memory usage of CSV exporter #19315

Description:

When exporting e.g. Live.getLastVisitsDetails with a result that contains a lot visits and some visits that have hundreds of actions, this can currently cause an out of memory problem, as iterating over all the data and extending the result array with thousands of empty keys, bloats up the array too much.

Did some local testing having a visit with more than 300 actions and a result set of only around 100 visits.

Before the changes of this PR the API export had a memory peak of around 73.762.528 Bytes
With the changes the peak went down to only 46.386.536 Bytes.

fixes #19158

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed