case_validate: Default to None rather than "none"

Defaulting to the string `"none"` has a functional consequence in
downstream pySHACL operations, as encoded at the time of this writing
(pySHACL version 0.23.0).  `inference` in the `pyshacl.validate(...)`
arguments is propagated down two code paths that both impact the
validation operation, but that use different default values.  The
`--metashacl` (SHACL-SHACL validation graph for reviewing shape syntax)-
aligned code path defaults to `rdfs` inference, and the `Validator`
class defaults to the string `"none"`.

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

case_utils/case_validate/__init__.py

@@ -136,12 +136,12 @@ def main() -> None:
         action="store_true",
         help="(As with pyshacl CLI) Allow import of sub-graphs defined in statements with owl:imports.",
     )
+    # NOTE: The "ontology graph" in the --inference help is the mix of the SHACL shapes graph and OWL ontology (or RDFS schema) graph.
     parser.add_argument(
         "-i",
         "--inference",
         choices=("none", "rdfs", "owlrl", "both"),
-        default="none",
-        help='(As with pyshacl CLI) Choose a type of inferencing to run against the Data Graph before validating. Default is "none".',
+        help='(As with pyshacl CLI) Choose a type of inferencing to run against the Data Graph before validating. The default behavior if this flag is not provided is to behave as "none", if not using the --metashacl flag.  The default behavior when using the --metashacl flag will apply "rdfs" inferencing to the ontology graph, but the data graph will still have no inferencing applied.  If the --inference flag is provided, it will apply to both the ontology graph, and the data graph.',
     )
     parser.add_argument(
         "-m",