When attempting to display actions that can be taken, having no actions caused error

[netniV]

Describe the bug
If an array is used to build up the actions to be performed, and the array is empty or the variable is null, draw_actions_dropdown() will fail and cause a backtrace.

To Reproduce
Steps to reproduce the behavior:

1. include 'lib/html.php'
2. call draw_actions_dropdown(NULL)
3. See error

Expected behavior
If there are no actions to be performed, or the variable isn't primed, draw_actions_dropdown() should simply exit without error.

Additional context

2019/02/26 12:24:28 - CMDPHP PHP ERROR Backtrace: (CactiShutdownHandler())
2019/02/26 12:24:28 - ERROR PHP ERROR: Uncaught Error: Unsupported operand types in /usr/share/cacti/develop/lib/html.php:1377 
Stack trace:
 #0 /usr/share/cacti/develop/plugins/webseer/webseer.php(430): draw_actions_dropdown(NULL)
 #1 /usr/share/cacti/develop/plugins/webseer/webseer.php(46): list_urls()
 #2 {main} thrown in file: /usr/share/cacti/develop/lib/html.php on line: 1377

This was caused by a variable being used globally, that wasn't properly registering as a global variable. As such, when the edit form was being displayed, the above stack trace appeared because the global variable was thought to be empty.

[netniV]

This has now been committed to the development code.