fix: undefined method closed? for nil

When Errno::EMFILE Too many open files hit, IO.pipe cannot return descriptors, check that it's not nil before closing.

Author: route

lib/ferrum/browser/process.rb

@@ -178,7 +178,7 @@ def ws_url=(url)
 
       def close_io(*ios)
         ios.each do |io|
-          io.close unless io.closed?
+          io.close if io && !io.closed?
         rescue IOError
           raise unless RUBY_ENGINE == "jruby"
         end