Fix EZP-23195: An empty line at the beginning of a Text block will be lost

JIRA: https://jira.ez.no/browse/EZP-23195

# Description

As explained in the issue, if a text block value starts with a new line, this one will be lost after the form validation (actually when the form is displayed the second time). It seems like this is happening because [the HTML documents are parsed like a SGML document](http://www.w3.org/TR/html401/appendix/notes.html#notes-line-breaks) so the first line break is ignored. Oddly, the last line is not removed and I was not able to find any relevant info in the HTML5 specifications...

Note: the same is valid for PlatformUI and is fixed in https://github.com/ezsystems/PlatformUIBundle/pull/102

# Test

manual tests in Firefox, Chrome and IE10.