Support docx files without footnotes

[brent-p]

Firstly thanks for this awesome gem!!
I ran into a small issue on some of the docx files I tried to parse where they didn't have any footnotes. So I have added some small checks to prevent runtime errors when trying to parse these files.

Side note:
I am not sure if this should be a separate pull request because I don't know a lot about ruby conventions but I had some small issues trying to get this gem to work and solved them by adding the following to my gemfile:

gem 'rubyzip', '< 1.0.0'
gem 'publishr',
gem 'docx_converter', '~> 1.0'

The newer rubyzip no longer supports this line in lib/docx_converter.rb:
require 'zip/zipfilesystem'
So setting it to require an older version fixed this issue.

The publisher gem is required in lib/docx_converter.rb but the publisher gem is not mentioned in the gemspec file, should this be added to the gemspec? or is it expected that users should add this manually?

When I tried adding just gem 'docx_converter' to my side project, bundler complained that it couldn't find the gem docx_converter but changing it to use a specific version as mentioned above fixed that problem so I am not sure if this should be updated in the readme.

Thanks again for this gem and please go easy on me as this is my very first pull request :)
Brent

[Jack12816]

I can confirm both issues. (footnotes, installation)
We should also take care of the rmagick requirement. (lib/docx_converter)

[DEPRECATION] requiring "RMagick" is deprecated. Use "rmagick" instead