Cast AbstractPicoPlugin::$dependsOn to array

Plugin devs could come up with the idea of setting AbstractPicoPlugin::$dependsOn to a string (single dependency) or null (no dependencies)

lib/AbstractPicoPlugin.php

@@ -187,7 +187,7 @@ abstract class AbstractPicoPlugin implements PicoPluginInterface
      */
     public function getDependencies()
     {
-        return $this->dependsOn;
+        return (array) $this->dependsOn;
     }
 
     /**