Error 500 loading merge requests due to `nil` parameter in Repository#merge_base_commit

https://sentry.gitlap.com/gitlab/gitlabcom/issues/30592/

TypeError: wrong argument type nil (expected String)
  lib/gitlab/metrics/instrumentation.rb:156:in `merge_base'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  lib/gitlab/metrics/instrumentation.rb:156:in `block in merge_base'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  lib/gitlab/metrics/method_call.rb:23:in `measure'
    retval = yield
  lib/gitlab/metrics/instrumentation.rb:156:in `merge_base'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  lib/gitlab/git/repository.rb:456:in `merge_base_commit'
    rugged.merge_base(from, to)
...
(187 additional frame(s) were not displayed)

ActionView::Template::Error: wrong argument type nil (expected String)

ActionView::Template::Error: wrong argument type nil (expected String)

Zendesk:

• https://gitlab.zendesk.com/agent/tickets/81899
• https://gitlab.zendesk.com/agent/tickets/82146