fix: handle None encoder_conf in FSMN-VAD init

Prevents TypeError when encoder_conf is not provided.
Cherry-picked from #2958.

Author: LauraGPT

funasr/models/fsmn_vad_streaming/model.py

@@ -397,6 +397,8 @@ def __init__(
         self.vad_opts = VADXOptions(**kwargs)
 
         encoder_class = tables.encoder_classes.get(encoder)
+        if encoder_conf is None:
+            encoder_conf = {}
         encoder = encoder_class(**encoder_conf)
         self.encoder = encoder
         self.encoder_conf = encoder_conf