undefined method `include?' for nil:NilClass

## I want to report a bug:

### What is the current behavior?

```
bundle exec jekyll algolia
Configuration file: /root/site/_config.yml
Processing site...
jekyll 4.3.1 | Error: undefined method `include?' for nil:NilClass

translate_posts = !site.config['exclude_from_localizations'].include?("_posts")
^^^^^^^^^
/root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-multiple-languages-plugin-1.8.0/lib/jekyll-multiple-languages-plugin.rb:251:in `read_posts': undefined method `include?' for nil:NilClass (NoMethodError)

translate_posts = !site.config['exclude_from_localizations'].include?("_posts")
^^^^^^^^^
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-4.3.1/lib/jekyll/reader.rb:91:in `retrieve_posts'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-4.3.1/lib/jekyll/reader.rb:76:in `read_directories'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-4.3.1/lib/jekyll/reader.rb:16:in `read'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-4.3.1/lib/jekyll/site.rb:181:in `read'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-algolia-1.7.1/lib/jekyll/algolia/overwrites/jekyll-algolia-site.rb:23:in `process'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-algolia-1.7.1/lib/jekyll-algolia.rb:82:in `run'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-algolia-1.7.1/lib/jekyll/commands/algolia.rb:38:in `block (2 levels) in init_with_program'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `block in execute'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `each'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `execute'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/program.rb:44:in `go'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary.rb:21:in `program'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/jekyll-4.3.1/exe/jekyll:15:in `'
from /root/3.1.2/bin/jekyll:25:in `load'
from /root/3.1.2/bin/jekyll:25:in `'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/cli/exec.rb:58:in `load'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/cli/exec.rb:58:in `kernel_load'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/cli/exec.rb:23:in `run'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/cli.rb:486:in `exec'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/cli.rb:31:in `dispatch'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/cli.rb:25:in `start'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/exe/bundle:48:in `block in '
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/lib/bundler/friendly_errors.rb:120:in `with_friendly_errors'
from /root/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.22/exe/bundle:36:in `'
from /root/3.1.2/bin/bundle:25:in `load'
from /root/3.1.2/bin/bundle:25:in `'
```

### What is your expected behavior?

I would expect it to create index the pages into algolia.

### Git repository to reproduce the issue:
https://github.com/hellotext/help

### Ruby version used:
ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-darwin21]

### Jekyll version used:
jekyll 4.3.1

Start with jekyll-multiple-languages-plugin.rb:251, the call shown in the stack trace, and reproduce it with bundle exec jekyll algolia using the linked repository and its _config.yml. Done means the command completes indexing the pages into Algolia without the NoMethodError.

Written by the indexing model from the issue text.