Bugfix: Percentage formatter failed when total was 0 or unknown

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Github Issue URLs:
  * https://github.com/jfelchner/ruby-progressbar/issues/75
Actions:
  * Closes #75
Branch:        bug/fix-percentage-formatter-when-total-is-unknown
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Merge branch 'bug/fix-percentage-formatter-when-total-is-unknown-closes-75'