Fixes bug when nblocks is empty list

Author: robvandijk

pymupdf4llm/pymupdf4llm/helpers/multi_column.py

@@ -157,6 +157,9 @@ def clean_nblocks(nblocks):
             if bb0 == bb1:
                 del nblocks[i]
 
+        if len(nblocks) == 0:
+            return nblocks
+
         # 2. repair sequence in special cases:
         # consecutive bboxes with almost same bottom value are sorted ascending
         # by x-coordinate.
@@ -452,6 +455,8 @@ def join_rects_phase3(bboxes, path_rects, cache):
 
     # do some elementary cleaning
     nblocks = clean_nblocks(nblocks)
+    if len(nblocks) == 0:
+        return nblocks
 
     # several phases of rectangle joining
     nblocks = join_rects_phase1(nblocks)