Avoid array conversion warning when daily events are disabled
When daily events are disabled then the "time" entry within the "end_time" entry will be an empty string. This evaluates to false which means the "end_time" entry will be left untouched.
This means that the value will be left as is and passed along.
As a result of this Drupal will try to save the end time with the value ["time"=>""] instead of an empty string.
To avoid this the !empty() check is replaced with isset() which will evaluate to true if "time" is an empty string.
Closes #3416436