fix: Allow num_buckets to be equal as min_bucket

[Koc]

Summary

We have next configuration for Multibucket Object Store

'objectstore_multibucket' => [
        'class' => 'Object\\Storage\\Backend\\Class',
        'arguments' => [
                'min_bucket' => 100,
                'num_buckets' => 100,
        ],
],

This needed to separate system files from a user files. So, system files will be stored in nextcloud-0 and user files in nextcloud-100. But the problem is such configuration not works as expected due to division by zero. This PR aims to fix that.

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)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)