Fix: exception layout_type in is_caption (#9028)

### What problem does this PR solve?

Exception layout_type in is_caption.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

deepdoc/vision/table_structure_recognizer.py

@@ -97,7 +97,7 @@ class TableStructureRecognizer(Recognizer):
             r"[图表]+[ 0-9:：]{2,}"
         ]
         if any([re.match(p, bx["text"].strip()) for p in patt]) \
-                or bx["layout_type"].find("caption") >= 0:
+                or bx.get("layout_type", "").find("caption") >= 0:
             return True
         return False