Script Server can raise unexpected warnings when 'arg_num_indexes' set but not found in data source

[cigamit]

Describe the bug
If you have a Script Server Data Query that attempts to implement arg_num_indexes, even though that query will return no rows, cacti does not attempt to emulate using the index count returned by the index query, but instead fails.

To Reproduce
Steps to reproduce the behavior:

1. Install the Mikrotik Plugin
2. Discover a Mikrotik device
3. Notice that the Mikrotik - Interfaces data query has zero rows

Expected behavior
Even though the 'arg_num_indexes' is defined in the XML file, we should not fail if it's unable to find any indexes.