fix: signinSilent fails when no user is available

This fixes an issue introduced in 1.6.0
(14ed108).
If there is no user available, signinSilent won't work because of the
following TypeError:

```
oidc-client.min.js:formatted:8872 Error during OIDC autologin TypeError: Cannot read property 'id_token' of null
    at oidc-client.min.js:formatted:8232
```

src/UserManager.js

@@ -158,7 +158,7 @@ export class UserManager extends OidcClient {
                 return this._useRefreshToken(args);
             }
             else {
-                args.id_token_hint = args.id_token_hint || (this.settings.includeIdTokenInSilentRenew && user.id_token);
+                args.id_token_hint = args.id_token_hint || (this.settings.includeIdTokenInSilentRenew && user && user.id_token);
                 return this._signinSilentIframe(args);
             }
         });

test/unit/UserManager.spec.js

@@ -108,6 +108,7 @@ describe("UserManager", function () {
 
                 navArgs.silentRequestTimeout.should.equal(123);
                 done();
+                return Promise.resolve()
             }
             subject.signinSilent();
         });
@@ -122,6 +123,7 @@ describe("UserManager", function () {
             subject._signin = function(args, nav, navArgs){
                 navArgs.silentRequestTimeout.should.equal(234);
                 done();
+                return Promise.resolve()
             }
             subject.signinSilent({silentRequestTimeout:234});
         });
@@ -136,9 +138,21 @@ describe("UserManager", function () {
             subject._signin = function(args, nav, navArgs){
                 args.prompt.should.equal("foo");
                 done();
+                return Promise.resolve()
             }
             subject.signinSilent({prompt:"foo"});
         });
+
+        it("should work when having no User present", function(done) {
+            settings.silent_redirect_uri = "http://client/silent_callback";
+            subject = new UserManager(settings);
+
+            subject._signin = function(){
+                done();
+                return Promise.resolve()
+            }
+            subject.signinSilent({prompt:"foo"});
+        })
     });
 
 });