yield `home_dir` when HOMEDIR is not falsy only.
https://github.com/ruby/ruby/blob/d0f41aa2382612022162d033ddb3d9c9cc62099e/lib/rdoc/ri/paths.rb#L43
```
    yield home_dir,    :home   if home and HOMEDIR
```

https://rubyci.org/logs/rubyci.s3.amazonaws.com/archlinux/ruby-master/log/20200426T033503Z.fail.html.gz
```
  1) Failure:
TestRDocRIPaths#test_class_each [/home/chkbuild/build/20200426T033503Z/ruby/test/rdoc/test_rdoc_ri_paths.rb:54]:
<nil> expected but was
<"/home/naruse/chkbuild/tmp/build/20200426T033503Z/tmp/test_rdoc_ri_paths_1033468/doc/nodoc-1.0/ri">.

  2) Failure:
TestRDocRIPaths#test_class_raw_path [/home/chkbuild/build/20200426T033503Z/ruby/test/rdoc/test_rdoc_ri_paths.rb:131]:
<nil> expected but was
<"/home/naruse/chkbuild/tmp/build/20200426T033503Z/tmp/test_rdoc_ri_paths_1033468/doc/rake-10.0.1/ri">.

  3) Failure:
TestRDocRIPaths#test_class_raw_path_extra_dirs [/home/chkbuild/build/20200426T033503Z/ruby/test/rdoc/test_rdoc_ri_paths.rb:141]:
<nil> expected but was
<"/home/naruse/chkbuild/tmp/build/20200426T033503Z/tmp/test_rdoc_ri_paths_1033468/doc/rake-10.0.1/ri">.
```