Fix: treat empty access token string as undefined

Fix AccessToken's method tokenIdForRequest to treat an empty string
as if no access token was provided.

This is needed to accomodate the changes made in
loopback-datasource-juggler@2.56.0.

common/models/access-token.js

@@ -209,6 +209,11 @@ module.exports = function(AccessToken) {
       if (typeof id === 'string') {
         // Add support for oAuth 2.0 bearer token
         // http://tools.ietf.org/html/rfc6750
+
+        // To prevent Error: Model::findById requires the id argument
+        // with loopback-datasource-juggler 2.56.0+
+        if (id === '') continue;
+
         if (id.indexOf('Bearer ') === 0) {
           id = id.substring(7);
           // Decode from base64

test/access-token.test.js

@@ -200,6 +200,16 @@ describe('loopback.token(options)', function() {
         .end(done);
     });
 
+  it('should generate a 401 on a current user literal route with empty authToken',
+    function(done) {
+      var app = createTestApp(null, done);
+      request(app)
+        .get('/users/me')
+        .set('authorization', '')
+        .expect(401)
+        .end(done);
+    });
+
   it('should generate a 401 on a current user literal route with invalid authToken',
     function(done) {
       var app = createTestApp(this.token, done);