Prevent crash when comparing ill-typed numeric types.

[ashleysommer]

This fixes comparisons that look like:

if Literal("1234", datatype="xsd:integer") > Literal("hello", datatype="xsd:integer")

Both of the dataypes are in the _NUMERIC_LITERAL_TYPES list, so they enter the fast-path comparison.
The problem is the second literal is not a valid integer, so it has the value None.
That throws a comparison error that is uncaught by RDFLib, causing the application to crash.

If this seems like something that is pretty common and should be picked up sooner, I think its because this error only occurs when rdflib.NORMALIZE_LITERALS is False. When rdflib normalizes the literals, it fixes the value, so this does not happen. I found this when testing some PySHACL code sections which run with rdflib.NORMALIZE_LITERALS=False.

Luckily since rdflib introduced the concept of ill_typed literals, we can use that to restrict the fast-path to only those numeric literals that are not ill-typed.

[coveralls]

coverage: 90.302%. remained the same
when pulling caba1c8 on compare_numeric
into 82c2421 on main.