Aeroval: heatmap data is also wrong if there's all NaN's in a season (related #639)

[jgriesfeller]

Basically the same as #639 but for heatmap data:

The loops are basically the same at the heatmap data computation as at the map data computation:

pyaerocom/pyaerocom/aeroval/coldatatojson_helpers.py

Lines 1117 to 1126 in e8f0c45

	for freq, coldata in data.items():
	output[freq] = hm_freq = {}
	use_dummy = True if coldata is None else False
	for regid, regname in region_ids.items():
	hm_freq[regname] = {}
	for per in periods:
	for season in seasons:
	perstr = f"{per}-{season}"
	if use_dummy:
	stats = stats_dummy

pyaerocom/pyaerocom/aeroval/coldatatojson_helpers.py

Line 1119 in e8f0c45

use_dummy = True if coldata is None else False

needs to be moved into the seasons loop