Handle null multichoice value
Handle null multichoice value

There is some data in the production database where the value of a
multichoice field audit is the string "null". Adding a None check to
the `clean_value` function prevents an exception where code further
down in the function expects the value to be iterable.

maurizi/otm-core,maurizi/otm-core,maurizi/otm-core,maurizi/otm-core