Fix validation message

The check was checking for an empty string in `matrix_jitsi_prosody_auth_internal_accounts`,
which is unlikely to happen. We should check for an empty list instead.

The check was not validating username/password values, so telling the user that they need a non-empty
username/password is misleading. It was merely checking if there's at least one entry in the list.

This patch adjusts the check and message accordingly.

roles/matrix-jitsi/tasks/validate_config.yml

@@ -23,16 +23,14 @@
 - name: Fail if a Jitsi internal authentication account is not defined
   fail:
     msg: >-
-      Each Jitsi user in `matrix_jitsi_prosody_auth_internal_accounts` needs to have a non-empty `username`/`password`.
-
-      Please ensure you have defined at least one user and password when using internal authentication.
+      At least one Jitsi user needs to be defined in `matrix_jitsi_prosody_auth_internal_accounts` when using internal authentication.
       
       If you're setting up Jitsi for the first time, you may have missed a step.
       Refer to our setup instructions (docs/configuring-playbook-jitsi.md).
   when:
     - matrix_jitsi_enable_auth|bool
     - matrix_jitsi_auth_type == 'internal'
-    - matrix_jitsi_prosody_auth_internal_accounts == ''
+    - matrix_jitsi_prosody_auth_internal_accounts|length == 0
 
 
 - name: (Deprecation) Catch and report renamed settings