Fix crash when username is null in mms auth

Fixes #2850
Closes #2863

// FREEBIE

src/org/thoughtcrime/securesms/mms/LegacyMmsConnection.java

@@ -241,7 +241,7 @@ public abstract class LegacyMmsConnection {
     }
 
     public boolean hasAuthentication() {
-      return !TextUtils.isEmpty(username) || !TextUtils.isEmpty(password);
+      return !TextUtils.isEmpty(username);
     }
 
     public String getUsername() {