Optimization: don't load group if not needed

Vasilii Iakliushinrequested to merge
482942_optimize_group_call into master

What does this MR do and why?

Contributes to #482942 (closed)

Problem

belongs_to without optional: true executes a DB query even when namespace_id is NULL.

Solution

Use optional: true for group relation of ProtectedBranch.

Edited by Vasilii Iakliushin

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