Patch EmptyTemplateHandler#call for Rails 6

In Rails 6 the API for template handlers is changing, a template handler
must now take two arguments [1], the template and the source.

I have attempted to retain support for older versions of Rails by
providing a default source of `nil`.

[1] https://www.github.com/rails/rails/commit/28f88e0074

lib/rspec/rails/view_rendering.rb

@@ -102,7 +102,7 @@ def find_templates(*args)
 
       # @private
       class EmptyTemplateHandler
-        def self.call(_template)
+        def self.call(_template, _source = nil)
           ::Rails.logger.info("  Template rendering was prevented by rspec-rails. Use `render_views` to verify rendered view contents if necessary.")
 
           %("")