coretasks: fix SASL EXTERNAL without password set

sopel/coretasks.py

@@ -112,7 +112,7 @@ def _handle_sasl_capability(
         return plugin.CapabilityNegotiation.ERROR
     # Check SASL configuration (password is required)
     password, mech = _get_sasl_pass_and_mech(bot)
-    if not password:
+    if mech != "EXTERNAL" and not password:
         raise config.ConfigurationError(
             'SASL authentication required but no password available; '
             'please check your configuration file.',