API crahes upon queries with empty parts

[hermannsblum]

The API crahes in case that a where query contains an empty part, e.g. where={"$and":[{"$or":[]},{"group":"5ac933284db3bd0001e9f2a9"}]}. The same query succeeds if the part {"$or":[]} is removed.

Easier query to reproduce (no auth required):

https://api-dev.amiv.ethz.ch/events?where=%7B%22%24and%22%3A%5B%7B%22%24or%22%3A%5B%5D%7D%2C%7B%22_id%22%3A%225b042d6f872eae0001b459d6%22%7D%5D%7D

vs

https://api-dev.amiv.ethz.ch/events?where=%7B%22%24and%22%3A%5B%7B%22_id%22%3A%225b042d6f872eae0001b459d6%22%7D%5D%7D

The expected behaviour should be that the query returns a 4xx error code because it is ill defined.