number_to_phone broken with empty number and area_code: true

This raises a "TypeError Exception: can't convert nil into String" exception:

    number_to_phone "", area_code: true

These both work (returns ""):

    number_to_phone "", area_code: false
    number_to_phone ""

The reason for the exception is that the #gsub! on line 73 of actionpack/lib/action_view/helpers/number_helper.rb returns nil when there is no match and String#<< is called with a nil argument.

The fix is easy. Replace line 50:

    return nil if number.nil?

...with:

    return nil if number.blank?