Fix binary encoding error on MR diffs

CharlockHolmes does not return an encoding for binary strings and hence we fail when we can't detect this encoding. This prevents the error to happen. A better fix is in https://github.com/brianmario/charlock_holmes/pull/113

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/32670

• Changelog entry added, if necessary
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