Change is_inorganic() method to default to None if no molecular formula

[Mike Dewhirst]

Until now, the auto-note created when figuring out which studies/test results are needed for asserting a chemical is not PBT relied upon is_inorganic() as the first "gate" to prove the assertion. If is_inorganic() returns True it gets used.

When testing a chemical with no molecular formula detected and none entered, returning True is incorrect.

This ticket adjust the return value to None if there is no formula and thereby passes the buck to subsequent tests.

The success test is seeing the following (aqueous solubility) "gate" or subsequent gates being triggered for PBT assertions.