feat: skip encoder_attention_mask in guider when mask is None

After batch=1 trimming sets prompt_attention_mask=None, prevent
passing None masks to the guider even when use_attention_mask=True.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pipeline_motif_video.py

@@ -1250,7 +1250,7 @@ class MotifVideoPipeline(DiffusionPipeline):
                 guider_inputs = {
                     "encoder_hidden_states": (prompt_embeds, negative_prompt_embeds),
                 }
-                if use_attention_mask:
+                if use_attention_mask and prompt_attention_mask is not None:
                     guider_inputs["encoder_attention_mask"] = (
                         prompt_attention_mask,
                         negative_prompt_attention_mask,