Make Credentials.homeServer optional because it is deprecated (#1443)

It is never used currently in RiotX, but if homeserver does not sent it, it would crash.

CHANGES.md

@@ -12,6 +12,7 @@ Improvements 🙌:
 Bugfix 🐛:
  - Switch theme is not fully taken into account without restarting the app
  - Temporary fix to show error when user is creating an account on matrix.org with userId containing only digits (#1410)
+ - Make Credentials.homeServer optional because it is deprecated (#1443)
 
 Translations 🗣:
  -

matrix-sdk-android/src/main/java/im/vector/matrix/android/api/auth/data/Credentials.kt

@@ -45,7 +45,7 @@ data class Credentials(
          * @Deprecated. Clients should extract the server_name from user_id (by splitting at the first colon)
          * if they require it. Note also that homeserver is not spelt this way.
          */
-        @Json(name = "home_server") val homeServer: String,
+        @Json(name = "home_server") val homeServer: String?,
         /**
          * ID of the logged-in device. Will be the same as the corresponding parameter in the request, if one was specified.
          */