fix #559: don't display the share view with one blog and text

* fix a NPE when user doesn't have visible blog
* if the user isn't signed in, redirect it to the login screen, if he's signed and have no visible blog, display a toast error and finish the share activity
* move isSignedIn to Worpress.java
Merge pull request #565 from wordpress-mobile/issue/559-unnecessary-share-view

fix #559: don't display the share view with one blog and text