[BUGFIX] EM: Handle missing dependencies in emconf

ext_emconf.php may lack the dependencies array. This will cause
a PHP warning in a foreach loop.

Fix this by checking for the type first.

Resolves: #58640
Releases: 6.2
Change-Id: Ifb04268e04f5349282f895f6b3d7354dbc45affc
Reviewed-on: https://review.typo3.org/29947
Reviewed-by: Christian Kuhn
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring