A null property_value isn't allowed in Filter

[masojus]

Since the API does allow for a given property to be null and pass an exists filter with a value of true, and of course a property can have the string value 'null', there is a meaningful distinction here that needs to be addressed.

See here where we can fix this and add tests.

Note that the .NET SDK has the same bug.

We could be more explicit about which filters are actually nullable and proactively warn client code with exceptions if trying to, say, set the property_value to null for an exists filter when only true and false are acceptable values. However, we don't do any of that type of validation right now on the Filter.propertyValue so that might be out of scope, and at a minimum we should accept null.