[Positioning] Timestamp: update operator bool()

Test tv_sec and tv_usec independently instead of using operator
uint64_t().

owlps-positioning/src/timestamp.hh

@@ -228,7 +228,7 @@ inline bool Timestamp::operator>=(const uint64_t source) const
 
 inline Timestamp::operator bool() const
 {
-  return static_cast<uint64_t>(*this) > 0 ;
+  return timestamp.tv_sec > 0 || timestamp.tv_nsec > 0 ;
 }