API server error when requesting items that don't exist

[jkeifer]

What should be a 404 ends up being a 500 with a traceback like

[ERROR] TypeError: 'NoneType' object is not subscriptable
Traceback (most recent call last):
  File "/var/task/lambda_function.py", line 161, in lambda_handler
    item = statedb.dbitem_to_item(statedb.get_dbitem(payload_id))
  File "/var/task/cirrus/lib2/statedb.py", line 570, in dbitem_to_item
    state, updated = dbitem["state_updated"].split("_")

That statedb.get_dbitem() call is typed to return a dict, but can also return None (https://github.com/cirrus-geo/cirrus-geo/blob/main/src/cirrus/lib2/statedb.py#L83). That we are indiscriminately passing the return value of that function into statedb.dbitem_to_item(), which expects an item dict, not None, is the basis for this exception.