empty string is causing NoMethodError rather than Ingreedy::ParseFailed

[matkoniecz]

I would expect

require 'ingreedy'

Ingreedy.parse('')

to throw Ingreedy::ParseFailed rather than NoMethodError

/home/mateusz/.gem/ruby/2.3.0/gems/ingreedy-0.1.0/lib/ingreedy/unit_variation_mapper.rb:31:in `unit_from_variation': undefined method `first' for nil:NilClass (NoMethodError)
    from /home/mateusz/.gem/ruby/2.3.0/gems/ingreedy-0.1.0/lib/ingreedy/ingreedy_parser.rb:55:in `convert_unit_variation_to_canonical'
    from /home/mateusz/.gem/ruby/2.3.0/gems/ingreedy-0.1.0/lib/ingreedy/ingreedy_parser.rb:39:in `parse'
    from /home/mateusz/.gem/ruby/2.3.0/gems/ingreedy-0.1.0/lib/ingreedy.rb:16:in `parse'