fix: Update project pull mirror to not pull protected_branches or branch_regex from state when not set

Giannis Kepasrequested to merge
gitlab-community/gitlab-org/terraform-provider-gitlab:6900-pull-mirror into main

What does this MR do and why?

Fixes gitlab_project_pull_mirror updates that fail on GitLab 19.5+ with 400 {error: only_mirror_protected_branches, mirror_branch_regex are mutually exclusive}.

Closes #6900 (closed)

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