fix: ignore undefined and null params on modifiers

[xolott]

Why?

Sometimes we have a logic like this:

let $set;

if(someCondition) {
    $set = {
        field: 'value',
    };
}
Collection.update(selector, { $set });

The Mongo driver can handle that modifier. It just ignores the $set operation when its value is null or undefined.

Fixes: #398

Changes

• Ignore operators with null or undefined values
• Added a test case for this scenario
• Added the isNil utility function
• Added a test case for the isNil utility function

[StorytellerCZ]

Strange that the tests didn't run.

[StorytellerCZ]

@xolott can you take a look on the tests, now that they have run and failed?