Fix 1144388

Fix needed to address https://bugs.launchpad.net/openquake/+bug/1144388

Details: when a probabilistic calculator (associated with a lognormal distribution) is called multiple times, we need to reset to ```epsilon_idx``` counter, otherwise we will end up in getting an ```IndexError``` exception.

Minor change: when no ground motion fields are passed to the calculators, we still return an empty result (empty list or empty matrix) but with the same type as for the non-trivial cases.