Fix an issue that an assert occurrs if user inputs a password whose length is less...

Fix an issue that an assert occurrs if user inputs a password whose length is less than the minimum required length. The root cause is that under such case the Form Browser will pass a null string (string ID = 0) to the Config Access callback. The callback handler does not handle such case correctly.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@8103 6f19259b-4bc3-4df7-8a09-765794883524