fix: allow empty handlers

Author: paquiteau

src/snake/core/engine/base.py

@@ -221,6 +221,9 @@ def __call__(
         else:
             sim_conf.TR_eff = sim_conf.seq.TR
 
+        if handlers is None:
+            handlers = []
+
         if any(h.__updates_sim_conf__ for h in handlers) and resample_early:
             self.log.warning(
                 "Handlers are updating the sim_conf, incompatible with resample_early"