postmoogle: update jija2 defaults

roles/matrix-bot-postmoogle/templates/env.j2

@@ -12,7 +12,7 @@ POSTMOOGLE_LOGLEVEL={{ matrix_bot_postmoogle_loglevel }}
 POSTMOOGLE_NOENCRYPTION={{ matrix_bot_postmoogle_noencryption }}
 POSTMOOGLE_NOOWNER={{ matrix_bot_postmoogle_noowner }}
 POSTMOOGLE_FEDERATION={{ matrix_bot_postmoogle_federation }}
-POSTMOOGLE_USERS={{ matrix_bot_postmoogle_users | default('') | join(' ') }}
-POSTMOOGLE_ADMINS={{ matrix_bot_postmoogle_users | default('') | join(' ') }}
+POSTMOOGLE_USERS={{ matrix_bot_postmoogle_users | default([]) | join(' ') }}
+POSTMOOGLE_ADMINS={{ matrix_bot_postmoogle_users | default([]) | join(' ') }}
 
 {{ matrix_bot_postmoogle_environment_variables_extension }}