[BUGFIX] Invalid argument supplied for foreach in ExtensionModelUtility

Dependecies might be given as empty string, e.g.
array (
	'depends' =>
		array (
			'typo3' => '4.5.0-6.2.99',
		),
	'conflicts' => '',
	'suggests' =>
		array (
		),
),

This is how the kickstarter used to generate it.

Resolves: #57207
Releases: 6.2
Change-Id: I314249f8883ee82fdfa9ee81aca7a526716ccade
Reviewed-on: https://review.typo3.org/28671
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Ernesto Baschny
Reviewed-by: Markus Klein
Tested-by: Markus Klein