Fix for notices "Trying to access array offset on value of type bool" in ReportsCron.php

[heavygale]

Proper check for success of query before accessing the result array.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

The following notices occure if there's no custom mail-template for e.g. trafficmaxpercent_subject available for the required language:

PHP Notice:  Trying to access array offset on value of type bool in /var/www/froxlor/lib/Froxlor/Cron/Traffic/ReportsCron.php on line 427
PHP Notice:  Trying to access array offset on value of type bool in /var/www/froxlor/lib/Froxlor/Cron/Traffic/ReportsCron.php on line 431

I succesfully tested the proposed changes on my own live Froxlor instance in the "Reaching your diskspace limit" use-case.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes