Queries with null don't work as expected #704

Version info

Angular: 2.2.3

Firebase: 3.6.1

AngularFire: 2.0.0-beta.6

How to reproduce these conditions

Stackoverflow: AngularFire2: query if child exists

Firebase Google Group: AngularFire2 query with null doesn't work

Steps to set up and reproduce Sample data and security rules

See Stackoverflow question above.

Expected behavior

Queries with given ordeByChild: "property" and endAt : null is expected to filter out lists where those poperties are null/don't exist.

Actual behavior

Whole list is returned with no filtering.

As @cartant pointed out, check could be done here just against undefined maybe and not to null.