Fix NoMethodError for Projects::BlobController#diff_lines

What does this MR do and why?

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1794354

Problem

NoMethodError: undefined method `empty?' for nil:NilClass (NoMethodError)

    return render_404 if diff_hunks.empty?

Code:

    diff_hunks = Gitlab::Diff::ViewerHunk.init_from_expanded_lines(
      presenter.diff_lines(with_positions_and_indent: true),
      bottom,
      closest_line_number
    )
    return render_404 if diff_hunks.empty?

source

ViewerHunk#init_from_expanded_lines returns nil instead of [].

Solution

Return [] to match expectation and resolve the error.

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