cpp-common/bt2/shared-object.hpp: make the optional part public

cpp-common/bt2/shared-object.hpp: make the optional part public

Make it possible to have an empty shared object instead of needing two
`bt2s::optional` (the `bt2::SharedObject` one and the user one).

Adding a default constructor to build an empty shared object and the
`bool` operator to check whether or not it's empty; similar to
`std::shared_ptr`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I646cf883eba47ff1058c95d1b027396ee713fa0b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11399
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>