[rb] fix order of operations bug

Author: titusfortner

rb/spec/rspec_matchers.rb

@@ -29,7 +29,7 @@
   end
 
   failure_message do
-    but_message = if @deprecations_found.empty? || @deprecations_found.nil?
+    but_message = if @deprecations_found.nil? || @deprecations_found.empty?
                     'no deprecations were found'
                   else
                     "instead these deprecations were found: [#{@deprecations_found.join(', ')}]"