[TASK] Add tests for ContentObjectRenderer::getData()

* Adds unittests
* Cleanup and fix some things along the way
* Some modes didn't allow to be executed without
  adding a dummy-value (e.g. "parameters")

Change-Id: I2b4ff89c828384c852e6fbf9f82c55e6ebbac281
Resolves: #50258
Releases: 6.2
Reviewed-on: https://review.typo3.org/22473
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn