added a bool operator to entity class

include/matador/sql/entity.hpp

@@ -33,6 +33,8 @@ public:
   pointer get() { return obj_.get(); }
   const value_type* get() const { return obj_.get(); }
 
+  operator bool() const { return obj_.get() != nullptr; } // NOLINT(*-explicit-constructor)
+
 private:
   std::shared_ptr<value_type> obj_;
 };