Update UCAMLOOKUP_... settings to behave correctly when unset

As noted in https://gitlab.developers.cam.ac.uk/uis/devops/iar/webapp/-/merge_requests/16#note_104837, the UCAMLOOKUP_... settings need tweaking to read

UCAMLOOKUP_USERNAME = os.environ.get('UCAMLOOKUP_USERNAME')
UCAMLOOKUP_PASSWORD = os.environ.get('UCAMLOOKUP_PASSWORD')