[BUGFIX] RTE: Empty paragraphs are not correctly transformed

Problem: Empty paragraphs entered in the RTE may get lost on way to
database if no block is found in content, as well as on way to the RTE
if followed by a block.
Solution: Fix both transformations and add multiple unit test
cases.

Releases: 4.5, 4.7, 6.0, 6.1
Resolves: #36904
Resolves: #26141

Change-Id: I6c5495fb668f29267bcdeb7b44814a4bb9dc0f96
Reviewed-on: https://review.typo3.org/11621
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Tested-by: Mario Rimann
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn