[BUGFIX] Prevent crash saving <a> without "href" attribute

Prevent a PHP error in the HTML parser when saving a content element
containing an <a> tag without "href" attribute by using an empty string
instead of null in the link resolver.

Resolves: #80268
Releases: master
Change-Id: I249f8ad13ebd0b2e9430868fdce287b2745ae9bc
Reviewed-on: https://review.typo3.org/52038


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Tested-by: Stephan Großberndt <stephan@grossberndt.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Gianluigi Martino <gmartino27@gmail.com>
Tested-by: Gianluigi Martino <gmartino27@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>