fix(ebpf): try to assert only when it is not nil

The normalizeTimeArg function was not checking if the value was nil
before asserting it as an uint64.

This also adds the type to the error message.

pkg/ebpf/processor_funcs.go

@@ -361,9 +361,13 @@ func (t *Tracee) normalizeTimeArg(argNames ...string) func(event *trace.Event) e
 			if arg == nil {
 				return errfmt.Errorf("couldn't find argument %s of event %s", argName, event.EventName)
 			}
+			if arg.Value == nil {
+				continue
+			}
+
 			argTime, ok := arg.Value.(uint64)
 			if !ok {
-				return errfmt.Errorf("argument %s of event %s is not of type uint64", argName, event.EventName)
+				return errfmt.Errorf("argument %s of event %s is not uint64, it is %T", argName, event.EventName, arg.Value)
 			}
 			arg.Value = time.BootToEpochNS(argTime)
 		}