Dropping of statistics doesn't work when "num_valid" is 0

[lewisblake]

Describe the bug
Please provide a clear and concise description of what the bug is.

• Pyaerocom version: 0.16.0 (with unreleased updates from main-dev)
• Computing platform: PPI
• Configuration file (if applicable): /lustre/storeB/users/lewisb/Python/Evaluations/aeroval/config/config_files/obsonly/cfg_obs-meteorology.py
• Error message (if applicable): NA, the run doesn't crash.

To Reproduce
Steps to reproduce the behavior:

1. cd /lustre/storeB/users/lewisb/Python/Evaluations/aeroval/config/config_files/obsonly
2. qsub run_obs-meteorology.sh
3. Check files in map

Expected behavior
All statistics provided in drop_stats should be dropped from all fields, not just those that have valid data points.

Screenshots
Not required.

Additional context
Dropping all the stats in all fields is important to reduce file size. The obs-only runs can be very large (previously 60G). They're now about 49G but we want to reduce all the unnecessary clutter from these files.