DX-914: Fix nil key

[asbjornu]

Fix for when the :plantuml key is nil to avoid the following error:

/usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/options.rb:61:in `extract_raise_errors': undefined method `key?' for nil:NilClass (NoMethodError)
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/options.rb:14:in `initialize'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:94:in `new'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:94:in `replace_needle'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:85:in `block in replace_needles'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:83:in `gsub'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:83:in `replace_needles'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:71:in `block (2 levels) in register_hook'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:70:in `each'
    from /usr/gem/ruby/2.7.0/gems/kramdown-plantuml-1.1.10/lib/kramdown-plantuml/jekyll_provider.rb:70:in `block in register_hook'

[codecov[bot]]

Codecov Report

Merging #76 (e9441c3) into main (26cdcb6) will increase coverage by 0.46%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   94.01%   94.48%   +0.46%     
==========================================
  Files          12       12              
  Lines         418      417       -1     
==========================================
+ Hits          393      394       +1     
+ Misses         25       23       -2     

Impacted Files	Coverage Δ
lib/kramdown-plantuml/options.rb	100.00% <100.00%> (+3.92%)	:arrow_up:

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[mergify[bot]]

Thank you @asbjornu for your contribution!