Return 'N/A' if category is nil

What does this MR do and why?

This MR updates the category name lookup logic to handle when the provided category is nil and instead of failing with a Ruby error, return N/A. This situation can happen when an MR has two changes with:

• One change matching a category, e.g. :docs
• Another change that doesn't match a category and so returns nil.

The list of categories is then [:docs, nil] which causes issues without this patch.

Closes gitlab-org/gitlab-development-kit#1663 (closed)

Without this change

From https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/3584098444:

--snip--
bundler: failed to load command: danger (/builds/gitlab-org/gitlab-development-kit/vendor/bundle/ruby/3.0.0/bin/danger)
/builds/gitlab-org/gitlab-development-kit/vendor/bundle/ruby/3.0.0/gems/gitlab-dangerfiles-3.6.4/lib/danger/plugins/internal/helper.rb:241:in `label_for_category': \e[31m (Danger::DSLError)
[!] Invalid `Dangerfile` file: \e[31m
[!] Invalid `Dangerfile` file: undefined method `start_with?' for nil:NilClass\e[0m
 #  from /builds/gitlab-org/gitlab-development-kit/vendor/bundle/ruby/3.0.0/gems/gitlab-dangerfiles-3.6.4/lib/danger/rules/simple_roulette/Dangerfile:68
 #  -------------------------------------------
 #    row = +"| #{reviewer_note} | #{maintainer_note} |"
 >    row.prepend("| #{helper.label_for_category(category)} ") if has_categories
 #    row
 #  -------------------------------------------
\e[0m
 #  from /builds/gitlab-org/gitlab-development-kit/vendor/bundle/ruby/3.0.0/gems/gitlab-dangerfiles-3.6.4/lib/danger/rules/simple_roulette/Dangerfile:68
 #  -------------------------------------------
 #   >   #  
 #  -------------------------------------------
	from /builds/gitlab-org/gitlab-development-kit/vendor/bundle/ruby/3.0.0/gems/gitlab-dangerfiles-3.6.4/lib/danger/rules/simple_roulette/Dangerfile:68:in `markdown_row_for_spins'
--snip--

With this change

MR acceptance checklist

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