fix: Use NewEvent to init Event struct (#220)

Existing code examples write to maps in Event without doing proper nil-check, assuming the maps are non-nil.

This reverts a subtle behavior change introduced in v0.6.0.

client.go

@@ -320,7 +320,8 @@ func (client *Client) eventFromException(exception error, level Level) *Event {
 		err = usageError{fmt.Errorf("%s called with nil error", callerFunctionName())}
 	}
 
-	event := &Event{Level: level}
+	event := NewEvent()
+	event.Level = level
 
 	for i := 0; i < maxErrorDepth && err != nil; i++ {
 		event.Exception = append(event.Exception, Exception{