Fix: cpp-common/bt2: don't pass a lib obj ptr when constructing a `CommonNullValue`

Fix: cpp-common/bt2: don't pass a lib obj ptr when constructing a `CommonNullValue`

When trying to use `CommonValue::asNull()`, we get:

    /home/smarchi/src/babeltrace/src/cpp-common/bt2/value.hpp:1423:55: error: no matching function for call to 'bt2::CommonNullValue<const bt_value>::CommonNullValue(<brace-enclosed initializer list>)'
     1423 |     return CommonNullValue<LibObjT> {this->libObjPtr()};
          |                                                       ^

The `CommonNullValue` constructor takes no parameter, so don't try to
pass one.

Change-Id: I653ab947fc36134c8767f39185a9a2cd85511b90
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12050
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>