Error 500 when loading an avatar that does not exist

https://sentry.gitlap.com/gitlab/gitlabcom/issues/110350/

NoMethodError: undefined method `[]=' for nil:NilClass
  lib/gitlab/git/blob.rb:104:in `find_entry_by_path'
    path[/^\/*/] = ''
  lib/gitlab/git/blob.rb:179:in `find_by_rugged'
    blob_entry = find_entry_by_path(repository, root_tree.oid, path)
  lib/gitlab/git/blob.rb:29:in `block in find'
    find_by_rugged(repository, sha, path, limit: MAX_DATA_DISPLAY_SIZE)
  lib/gitlab/gitaly_client.rb:232:in `block (2 levels) in migrate'
    yield is_enabled
  lib/gitlab/gitaly_client.rb:274:in `allow_n_plus_1_calls'
    yield
...
(135 additional frame(s) were not displayed)

NoMethodError: undefined method `[]=' for nil:NilClass