Handle a missing description gracefully

b5da040 introduced word_wrap which
assumes text is a string. However, this is not true and not every
resource type has a description.

Author: ekohl

lib/puppet-strings/markdown/base.rb

@@ -170,6 +170,7 @@ def private?
     end
 
     def word_wrap(text, line_width: 120, break_sequence: "\n")
+      return unless text
       text.split("\n").collect! do |line|
         line.length > line_width ? line.gsub(/(.{1,#{line_width}})(\s+|$)/, "\\1#{break_sequence}").strip : line
       end * break_sequence