When exporting a data query, an invalid column name error can sometimes be shown

[ddb4github]

Describe the bug

Export DataQuery show PHP Error: ERROR PHP NOTICE: Undefined index: fname in file: /var/www/html/cacti/lib/export.php on line: 346
CMDPHP PHP ERROR NOTICE Backtrace: (/templates_export.php[33]:form_save(), /templates_export.php[58]:get_item_xml(), /lib/export.php[1102]:data_input_method_to_xml(), /lib/export.php[346]:CactiErrorHandler())

To Reproduce

Steps to reproduce the behavior:

1. Go to Console->Import/Export-->Export Templates

2. Select "What would you like to export?"-->Data Query

3. Select "Device Template to Export"--> Any Data Query

4. Enable "Include Dependencies"

5. Select "Output Format" --> "Save File Locally"

6. Click "Export"

7. Check "Logs" tab

8. See Error

[TheWitness]

Nice catch @ddb4github. That's for the QA.