lib: Make bt_value_null a const pointer

We probably don't want anything to change the value of this pointer,
which points to the only instance of BT_VALUE_TYPE_NULL,
bt_value_null_instance.  Therefore, declare it as a constant value.

It also helps a little bit the Python bindings: if we copy the
declaration verbatim to the .i file, without const, bt_value_null is
available through the special "cvar" global object (and can be
modified!).  With const, it becomes read-only, so can be accessed
directly as native_bt.value_null.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>