os_system(): do not set up input stream for empty string #7951

Test failure:
test/functional/eval/system_spec.lua: "works with an empty string"
E5677: Error writing input to shell-command: EPIPE

ref https://github.com/neovim/neovim/pull/6558#issuecomment-361061035
ref #6554

test/functional/eval/system_spec.lua

@@ -260,6 +260,7 @@ describe('system()', function()
     end)
     it('works with an empty string', function()
       eq("test\n", eval('system("echo test", "")'))
+      eq(2, eval("1+1"))  -- Still alive?
     end)
   end)