NoMethodError: undefined method `gsub' for nil:NilClass for links in csv formatter

[shcyiza]

Issue

While parsing a file converting xlsx file to CSV I came accros this error:
NoMethodError: undefined method 'gsub' for nil:NilClass
/gems/ruby/2.7.0/gems/roo-2.8.3/lib/roo/formatters/csv.rb:77:in 'cell_to_csv'
I cannot share the file for privacy reason unfortunately but it looks like it's an easy fix.

It seems that unlike the formatter for strings, the links part of the code is not nil safe.
location: /lib/roo/formatters/csv.rb:77:in `cell_to_csv'
change proposal here: master...shcyiza:patch-1

System configuration

Roo version: 2.8.3

Ruby version: 2.7.2p137