Check for null & undefined values

lib/dao.js

@@ -407,6 +407,9 @@ DataAccessObject._coerce = function (where) {
         }
 
         var val = where[p];
+        if(val === null || val === undefined) {
+          continue;
+        }
         // Check there is an operator
         var operator = null;
         if ('object' === typeof val) {

test/loopback-dl.test.js

@@ -654,6 +654,14 @@ describe('DataAccessObject', function () {
     assert.deepEqual(where, {location: {near: {lng: 10, lat: 20}, maxDistance: 20}});
   });
 
+  it('should skip null or undefined values', function () {
+    where = model._coerce({date: null});
+    assert.deepEqual(where, {date: null});
+
+    where = model._coerce({date: undefined});
+    assert.deepEqual(where, {date: undefined});
+  });
+
 });
 
 describe('Load models from json', function () {