sink.ctf.fs: use bt2c::Logger

sink.ctf.fs: use bt2c::Logger

Use `bt2c::Logger` throughout the `sink.ctf.fs` code.

When printing C strings that may be NULL (e.g. result of
`bt_trace_get_name()`), we currently rely on glibc printf's behavior of
printing `(null)` when passed a NULL pointer for `%s`.  {fmt} doesn't
accept a nullptr `const char *`.  Add the `bt2c::maybeNull()` function
to emulate the existing behavior.

Change-Id: I6ab259c8db526a3b4c0de9295a4360180c56b398
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12241
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>