Small clarification on findAsset

- make sure returned result is either `Asset` or `Null` exactly.
- add info about returned type into api doc

lib/mincer/base.js

@@ -277,7 +277,7 @@ Base.prototype.contentTypeOf = function (pathname) {
 
 
 /**
- *  Base#findAsset(pathname[, options = {}]) -> Asset
+ *  Base#findAsset(pathname[, options = {}]) -> Asset|Null
  *  - pathname (String)
  *  - options (Object)
  *
@@ -288,7 +288,7 @@ Base.prototype.findAsset = function (pathname, options) {
 
   if (isAbsolute(pathname)) {
     if (!this.stat(pathname)) {
-      return;
+      return null;
     }
 
     logical_path = this.attributesFor(pathname).logicalPath;