Potential fix for #31 - define 'res' as empty list, when it's not being created due to empty SPL search results

[BartekZm]

Potential fix for #31 - define 'res' as empty list, when it's not being created due to empty SPL search results:
slack_alerts/bin:

(...)
def build_fields_attachment(payload):  
    res = payload.get('result', dict())                                        
    if not res:                                                                
       res = {}                                                  
    available_fields = list(res.keys()) 
(...)

It addresses the problem in my test environment in context of the issue #31 and I observed no impact on the behavior & additional for other sample alerts and how they're performed. Yet, please perform any additional tests when needed (I have no dev background & experience).