Check for array in config event subscriber

Problem/Motivation

This is a result of #3268058: Media sources should declare dependency on Provider Bucket
The config data might not contain any dependencies, or might not declare module dependencies. Though it's not good to have empty dependencies, the config event subscriber should check for an array before using `array_search` function. In PHP 8.1 (at least) it produces a fatal error.
The error message is: "TypeError: array_search(): Argument #2 ($haystack) must be of type array, null given in array_search()"

Steps to reproduce

Using php >= 8.1 attempt to set a media type config to have empty dependencies and observe the error.
`drush cset media.type.video dependencies '{}' --input-format=yaml`

Proposed resolution

Check for an array before searching within it.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None