Fix: bt2: correctly handle empty result of bt_field_enumeration_unsigned_get_mapping_...

Fix: bt2: correctly handle empty result of bt_field_enumeration_unsigned_get_mapping_labels

When calling `_EnumerationFieldClassConst.mappings_for_value()` and the
result is empty (no matching mappings), we get:

      File "/home/smarchi/build/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/field_class.py", line 240, in mappings_for_value
        return [self[label] for label in labels]
                                         ^^^^^^
    TypeError: 'NoneType' object is not iterable

Here's what happens:

 - When the input value matches no mappings, the library sets the array
   output parameter to `NULL`.
 - The SWIG argout typemap checks the array output parameter value to
   choose between returning a list or `None`.  So in this case, we return
   the tuple `(__BT_FUNC_STATUS_OK, None)`.
 - Back in the Python side, we try to iterate over `None`.

Fix this by checking the status in the typemap to see if the call
succeeded.

Add a corresponding test.

Change-Id: If1c7de435c27b9c37b67b66a7b8384480ac654e1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13385
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>