Fix failures of test/rdoc/test_rdoc_ri_paths.rb

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">.
```

Author: znz

test/rdoc/test_rdoc_ri_paths.rb

@@ -51,7 +51,7 @@ def test_class_each
 
     assert_equal RDoc::RI::Paths.system_dir,          path.shift
     assert_equal RDoc::RI::Paths.site_dir,            path.shift
-    assert_equal RDoc::RI::Paths.home_dir,            path.shift
+    assert_equal RDoc::RI::Paths.home_dir,            path.shift if RDoc::RI::Paths.home_dir
     assert_equal File.join(@nodoc.doc_dir, 'ri'),     path.shift
     assert_equal File.join(@rake_10.doc_dir, 'ri'),   path.shift
     assert_equal File.join(@rdoc_4_0.doc_dir, 'ri'),  path.shift
@@ -128,7 +128,7 @@ def test_class_raw_path
 
     assert_equal RDoc::RI::Paths.system_dir,        path.shift
     assert_equal RDoc::RI::Paths.site_dir,          path.shift
-    assert_equal RDoc::RI::Paths.home_dir,          path.shift
+    assert_equal RDoc::RI::Paths.home_dir,          path.shift if RDoc::RI::Paths.home_dir
     assert_equal File.join(@rake_10.doc_dir, 'ri'), path.shift
   end
 
@@ -138,7 +138,7 @@ def test_class_raw_path_extra_dirs
     assert_equal '/nonexistent',                    path.shift
     assert_equal RDoc::RI::Paths.system_dir,        path.shift
     assert_equal RDoc::RI::Paths.site_dir,          path.shift
-    assert_equal RDoc::RI::Paths.home_dir,          path.shift
+    assert_equal RDoc::RI::Paths.home_dir,          path.shift if RDoc::RI::Paths.home_dir
     assert_equal File.join(@rake_10.doc_dir, 'ri'), path.shift
   end