[rubygems/rubygems] Stop using `/dev/null` for silent ui for WASI platform

WASI doesn't guarantee that `/dev/null` is present.
So without this patch, we needed to mount host's `/dev` directory to WASI
guest process to avoid `ENOTCAPABLE` error while `require "bundler/setup"`

https://github.com/rubygems/rubygems/commit/e9187ab61f

lib/rubygems/user_interaction.rb

@@ -616,18 +616,11 @@ class Gem::SilentUI < Gem::StreamUI
   # The SilentUI has no arguments as it does not use any stream.
 
   def initialize
-    reader, writer = nil, nil
-
-    reader = File.open(IO::NULL, 'r')
-    writer = File.open(IO::NULL, 'w')
-
-    super reader, writer, writer, false
+    io = NullIO.new
+    super io, io, io, false
   end
 
   def close
-    super
-    @ins.close
-    @outs.close
   end
 
   def download_reporter(*args) # :nodoc:
@@ -637,4 +630,25 @@ class Gem::SilentUI < Gem::StreamUI
   def progress_reporter(*args) # :nodoc:
     SilentProgressReporter.new(@outs, *args)
   end
+
+  ##
+  # An absolutely silent IO.
+
+  class NullIO
+    def puts(*args)
+    end
+
+    def print(*args)
+    end
+
+    def flush
+    end
+
+    def gets(*args)
+    end
+
+    def tty?
+      false
+    end
+  end
 end

test/rubygems/test_gem_silent_ui.rb

@@ -113,4 +113,10 @@ class TestGemSilentUI < Gem::TestCase
     assert_empty out, 'No output'
     assert_empty err, 'No output'
   end
+
+  def test_new_without_dev_null
+    File.stub(:open, ->(path, mode) { raise Errno::ENOTCAPABLE if path == IO::NULL }) do
+      Gem::SilentUI.new
+    end
+  end
 end