CalcConfig does not handle empty IMT array

If an empty array is specified for the imt element of the hazard configuration, CalcConfig should either:

1. throw an error when hazard config is parsed, or
2. populate the empty array with the default set of IMTs

Currently, the calculation proceeds and throws NoSuchElementException with a message "Got more than one input Future failure." from the transforms.

• imt element is omitted: the default set of IMTs is used and calculation succeeds
• imt element is assigned an array of IMTs, ["PGA", "SA0P1", ...] -
• imt element is assigned an empty array, [] - calculation runs but throws exceptions