fix(s3): treat empty sse_c_key as not set

[st3iny]

• Resolves: # none

Summary

Setting the sse_c_key to an empty string enables encryption despite no key actually being available.

Have a look at the following usage:

server/lib/private/Files/ObjectStore/S3ConnectionTrait.php

Lines 224 to 228 in abd7958

	$key = $this->getSSECKey();
	if ($key === null) {
	return [];
	}

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[solracsf]

Since we're using similar logic on the file (empty check) function could be simplified to:

protected function getSSECKey(): ?string {
	return !empty($this->params['sse_c_key']) ? $this->params['sse_c_key'] : null;
}

[juliusknorr]

Conflicts with #50326 where I asked for the adjustment 🙈

[st3iny]

Let's continue here: #50514