fix: Handle no result from RapidOcr reader (#558)

Signed-off-by: Christoph Auer <60343111+cau-git@users.noreply.github.com>

docling/models/rapid_ocr_model.py

@@ -118,6 +118,7 @@ class RapidOcrModel(BaseOcrModel):
                         del high_res_image
                         del im
 
+                        if result is not None:
                             cells = [
                                 OcrCell(
                                     id=ix,