Fix encoding issue exporting a project

Returns empty string on UTF8 conversion with U_STRING_NOT_TERMINATED_WARNING error

This is a workaround. Proper fix should be to return the converted string and ignore or warn about the error => https://github.com/brianmario/charlock_holmes/pull/114

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31186

Apart from the Import/Export, we also call this function from Gitlab::Git::Blame - not sure if there're other implications there. But I assume there shouldn't.

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