Add bool conversion operator to subscription_identifiers.

Reviewers: ivica

Reviewed By: ivica

Subscribers: korina

Differential Revision: https://repo.mireo.local/D27993

include/async_mqtt5/property_types.hpp

@@ -54,6 +54,10 @@ public:
 		return !empty();
 	}
 
+	explicit operator bool() const noexcept {
+		return !empty();
+	}
+
 	int32_t& operator*() noexcept {
 		return front();
 	}