gem install --document=ri,rdoc causes error with rdoc 7.0.3 with undefined method 'empty?' for an instance of RDoc::Markup::Paragraph

[mtasaka]

Looks like #1523 is not fixed even with rdoc 7.0.3
(And it seems that I have no permission to reopen the bug, so I resubmit the bug)

With rdoc 7.0.3, the above gem install --document=ri,rdoc causes errors for some gems, like

$ gem install -V hocon --document=ri,rdoc
....
....
Successfully installed hocon-1.4.0
Parsing documentation for hocon-1.4.0
Parsing sources...
100% [88/88]  lib/hocon/version.rb
Installing ri documentation for hocon-1.4.0
Installing aliki documentation for hocon-1.4.0
ERROR:  While executing gem ... (NoMethodError)
    undefined method 'empty?' for an instance of RDoc::Markup::Paragraph

    return '' unless first_comment && !first_comment.empty?
                                                    ^^^^^^^
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/code_object/class_module.rb:728:in 'RDoc::ClassModule#search_snippet'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/generator/aliki.rb:151:in 'RDoc::Generator::Aliki#build_class_module_entry'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/generator/aliki.rb:80:in 'block in RDoc::Generator::Aliki#build_search_index'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/generator/aliki.rb:77:in 'Array#each'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/generator/aliki.rb:77:in 'RDoc::Generator::Aliki#build_search_index'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/generator/aliki.rb:109:in 'RDoc::Generator::Aliki#write_search_index'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/generator/aliki.rb:32:in 'RDoc::Generator::Aliki#generate'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/rubygems_hook.rb:149:in 'block in RDoc::RubyGemsHook#document'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/rubygems_hook.rb:146:in 'Dir.chdir'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/rubygems_hook.rb:146:in 'RDoc::RubyGemsHook#document'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/rubygems_hook.rb:205:in 'RDoc::RubyGemsHook#generate'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/rubygems_hook.rb:62:in 'block in RDoc::RubyGemsHook.generate'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/rubygems_hook.rb:61:in 'Array#each'
	/home/tasaka1/.local/share/gem/ruby/gems/rdoc-7.0.3/lib/rdoc/rubygems_hook.rb:61:in 'RDoc::RubyGemsHook.generate'
	/usr/share/rubygems/rubygems/request_set.rb:313:in 'block in Gem::RequestSet#install_hooks'
	/usr/share/rubygems/rubygems/request_set.rb:312:in 'Array#each'
	/usr/share/rubygems/rubygems/request_set.rb:312:in 'Gem::RequestSet#install_hooks'
	/usr/share/rubygems/rubygems/request_set.rb:210:in 'Gem::RequestSet#install'
	/usr/share/rubygems/rubygems/commands/install_command.rb:207:in 'Gem::Commands::InstallCommand#install_gem'
	/usr/share/rubygems/rubygems/commands/install_command.rb:223:in 'block in Gem::Commands::InstallCommand#install_gems'
	/usr/share/rubygems/rubygems/commands/install_command.rb:216:in 'Array#each'
	/usr/share/rubygems/rubygems/commands/install_command.rb:216:in 'Gem::Commands::InstallCommand#install_gems'
	/usr/share/rubygems/rubygems/commands/install_command.rb:162:in 'Gem::Commands::InstallCommand#execute'
	/usr/share/rubygems/rubygems/command.rb:326:in 'Gem::Command#invoke_with_build_args'
	/usr/share/rubygems/rubygems/command_manager.rb:253:in 'Gem::CommandManager#invoke_command'
	/usr/share/rubygems/rubygems/command_manager.rb:194:in 'Gem::CommandManager#process_args'
	/usr/share/rubygems/rubygems/command_manager.rb:152:in 'Gem::CommandManager#run'
	/usr/share/rubygems/rubygems/gem_runner.rb:57:in 'Gem::GemRunner#run'
	/usr/bin/gem:12:in '<main>'