chain function with esri source fails

[andrewharvey]

machine/openaddr/cache.py

Lines 309 to 334 in 5ec8ff8

	def field_names_to_request(conform):
	''' Return list of fieldnames to request based on conform, or None.
	'''
	if not conform:
	return None
	fields = set()
	for k, v in conform.items():
	if k in attrib_types:
	if isinstance(v, dict):
	# It's a function of some sort?
	if 'function' in v:
	if v['function'] in ('join', 'format'):
	fields |= set(v['fields'])
	else:
	fields.add(v.get('field'))
	elif isinstance(v, list):
	# It's a list of field names
	fields |= set(v)
	else:
	fields.add(v)
	if fields:
	return list(filter(None, sorted(fields)))
	else:
	return None

Surfaced in https://s3.amazonaws.com/data.openaddresses.io/runs/664080/output.txt from openaddresses/openaddresses#4599

the above function fails for an ESRI source when using a chain function.