Remove nil check because blank? handles it

The check used to be session_name.strip.empty?, but I forgot to remove
the nil case when I converted to blank?.

Author: wvu

lib/msf/ui/console/command_dispatcher/core.rb

@@ -1477,7 +1477,7 @@ def cmd_sessions(*args)
       print(Serializer::ReadableText.dump_sessions(framework, :show_extended => show_extended, :verbose => verbose, :search_term => search_term))
       print_line
     when 'name'
-      if session_name.nil? || session_name.blank?
+      if session_name.blank?
         print_error('Please specify a valid session name')
         return false
       end