AndHowTestUtils.setAndHowCore should not throw error if current core is null

[eeverman]

Describe the bug
AndHowTestUtils.setAndHowCore#138 throw the error:
"Cannot set a new core when AndHow is uninitialized"

This error is actually only returned when AndHow is uninitialized and the new core is non-null. This method happily accepts a non-null core if AndHow is initialized - why would it complain if it is not??

Setting the core does break the state of AndHow a bit: Perhaps there was a Locator or a Config in process that does not match the new core - can that be sorted out??