Correct error checking for 0 fields requested in Grid-Stat.

[JohnHalleyGotway]

Jeff Hamilton at NOAA/GSD received a confusing error message from Grid-Stat:
ERROR : GridStatVxOpt::process_config() -> At least one output STAT or NetCDF type must be requested in "output_flag" or "nc_pairs_flag".

The underlying problem is that the list of fields to be verified was length 0. The error should have read:
ERROR : GridStatConfInfo::process_config() -> The number of verification tasks in "obs.field" (0) must be non-zero and match the number in "fcst.field" (0).

Correct this oversight in validating the configuration in both Grid-Stat and Ensemble-Stat.

[JohnHalleyGotway]

Fix this in master_v8.1 and develop. I originally thought it should be fixed in grid-stat and ensemble-stat. However, it's ok to have 0 verification fields in ensemble-stat. But in grid-stat, the number of vx tasks must be non-zero.