added _.isNull definition

lodash/lodash-tests.ts

@@ -629,6 +629,9 @@ result = <boolean>_.isNaN(NaN);
 result = <boolean>_.isNaN(new Number(NaN));
 result = <boolean>_.isNaN(undefined);
 
+result = <boolean>_.isNull(null);
+result = <boolean>_.isNull(undefined);
+
 var mergeNames = {
   'stooges': [
     { 'name': 'moe' },

lodash/lodash.d.ts

@@ -2443,6 +2443,13 @@ declare module _ {
 	**/
 	export function isNaN(value: any): boolean;
 
+	/**
+	* Checks if value is null.
+	* @param value The value to check.
+	* @return True if the value is null, else false.
+	**/
+	export function isNull(value: any): boolean;
+
 	/**
 	* Recursively merges own enumerable properties of the source object(s), that don't resolve 
 	* to undefined into the destination object. Subsequent sources will overwrite property 
@@ -2691,13 +2698,6 @@ declare module _ {
 	**/
 	export function isRegExp(object: any): boolean;
 
-	/**
-	* Returns true if the value of object is null.
-	* @param object Check if this object is null.
-	* @return True if `object` is null, otherwise false.
-	**/
-	export function isNull(object: any): boolean;
-
 	/**
 	* Returns true if value is undefined.
 	* @param object Check if this object is undefined.