#55 Allow IMAP connection with empty password

[Patrice Mandin]

Hello,

We got a customer that has an Exchange account with an empty password (internal, non public Exchange server).
Connecting without password is OK for calendar and contact (i.e. through CalDAV/CardDAV), however Davmail triggers an exception in the IMAP connector:

java.lang.NullPointerException
at davmail.util.IOUtil.decodeBase64(IOUtil.java:59)
at davmail.util.IOUtil.decodeBase64AsString(IOUtil.java:69)
at davmail.imap.ImapConnection.run(ImapConnection.java:139)

readClient() returns null when reading the password, so just checking this makes it working.
Patch attached