extractAttributeFromDict returns an empty dict if an attribute is missing

[abursavich]

SELECT returns an empty object when a field is missing because extractAttributeFromDict defaults the subtree to an empty dict instead of None.

cobra/cobras/common/algorithm.py

Lines 22 to 26 in 8c9d8b6

	fieldsComponents = attribute.split('.')
	for component in fieldsComponents:
	subtree = subtree.get(component, {})
	if subtree is None:
	return None