Fix for custom tag rendering

[IevaGraz]

After #1091 the behaviour when a render method of custom tags returns nil has changed. Before something like

 Liquid::Template.parse('foo {% custom_url%20SOME-CUSTOM-PARAM %}').render()

that depends on

 class CustomUrl < Liquid::Tag
     ...
    def render(_context)
      return unless something

      #build url
    end

Would return just foo .
After the change it returns foo Liquid error: internal
Because this line here throws TypeError: no implicit conversion of nil into String

[ianks]

Just checked this with our app's CI and it looks good. Thanks for the fix!