GPMixin: Fix goal is_empty check

If one of the targets is np.nan, and the other a Timeseries containing all np.nan (inactive), we would get an Exception thrown due to a size error. This is because this type of goal, although empty/inactive, as not being skipped in processing.

This commit fixes the goal.is_empty check, properly skipping such goals. We also add tests for the expected behavior of is_empty, e.g. that it still returns False when dealing with minimization goals (both targets np.nan).

Closes #1098 (closed)