[TASK] EXT:form - do not write placeholder attribute if empty

Do not write the placeholder html attribute if its value
is empty within the form editor.

Resolves: #80383
Releases: master
Change-Id: Icf514b49c73d3ccc9daa913dc55678bf7a57baa5
Reviewed-on: https://review.typo3.org/52113


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>