[BUGFIX] Add img longdescUrl only if available

According to the W3C standard (http://www.w3.org/TR/html-longdesc/#implementation),
longDesc should only be added if the attribute value is not empty.

The change adds an additional check to do so.

Resolves: #70685
Releases: master, 6.2
Change-Id: I4f61e61f04de81f881becee871edcc15d77deecd
Reviewed-on: https://review.typo3.org/44572


Reviewed-by: Daniel Gorges <daniel.gorges@b13.de>
Tested-by: Daniel Gorges <daniel.gorges@b13.de>
Reviewed-by: David Steeb <david.steeb@b13.de>
Tested-by: David Steeb <david.steeb@b13.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>