lib: make variant field class option name optional with MIP > 0

lib: make variant field class option name optional with MIP > 0

CTF2-SPEC-2.0 [1] calls for optionally named variant field class
option, therefore libbabeltrace2 should support this.

Before this patch, having a variant field class option name is a
precondition. This patch relaxes said precondition when the MIP version
is greater than zero.

Note that a structure field member class must still have a name.

[1]: https://diamon.org/ctf/CTF2-SPEC-2.0.html

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I7f6da1276f75e4c42f1e99c8d0ce1333c9bfee5f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8047
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12690