[TASK] Avoid empty tags with <f:format.html>

When using <f:format.html> you must not use empty
lines inside the tag as those will lead to empty p-tags.

It was already fixed with #45793 but reintroduced with #49908.

Change-Id: I83ec2e60938bd1efba369541c592093ff1b7ea00
Resolves: #52970
Reviewed-on: https://review.typo3.org/24914
Tested-by: Stefan Neufeind
Reviewed-by: Henrik Ziegenhain
Tested-by: Henrik Ziegenhain
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer