fix: fallback to field name if metadata is missing

[jozan]

in some cases there is no metadata available for some fields and it results field names to be None. this diff adds a fallback for such cases.

repro in python repl:

from fmiopendata.wfs import download_stored_query
import datetime as dt

now = dt.datetime.now()
start_time = now.strftime("%Y-%m-%dT00:00:00Z")
end_time = now.strftime("%Y-%m-%dT01:00:00Z")

model_data = download_stored_query(
	"fmi::forecast::edited::weather::scandinavia::point::multipointcoverage",
	args=[
		"starttime=" + start_time,
		"endtime=" + end_time,
		"place=Helsinki",
		"parameters=PoP",
	],
)

for key in model_data.data.keys():
	model_data.data[key]