[BUGFIX] Use config.extTarget as fallback for HMENU external links

If a page of type external link does not have a "target" defined
the fallback should be used. This behavior has been changed in
#96470 so the fallback is not
used anymore as an empty string is also respected as a valid value
but should not.

Resolves: #97964
Related: #96470
Releases: main, 11.5
Change-Id: I8270aec774a342151eee47000386546e3d5ac70c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75234


Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>

typo3/sysext/frontend/Classes/Typolink/AbstractTypolinkBuilder.php

@@ -176,7 +176,7 @@ abstract class AbstractTypolinkBuilder
             || in_array((string)$tsfe->config['config']['doctype'], ['xhtml_trans', 'xhtml_basic', 'html5'], true);
 
         $target = '';
-        if (isset($conf[$name])) {
+        if (isset($conf[$name]) && $conf[$name] !== '') {
             $target = $conf[$name];
         } elseif ($targetAttributeAllowed && !($conf['directImageLink'] ?? false)) {
             $target = $fallbackTarget;