Fix issue with attempting to deskew a blank page on Tesseract 5

Closes #868

src/ocrmypdf/_exec/tesseract.py

@@ -200,7 +200,9 @@ def get_deskew(
     except CalledProcessError as e:
         tesseract_log_output(e.stdout)
         tesseract_log_output(e.stderr)
-        if b'Empty page!!' in e.output:  # Not enough info for a skew angle
+        if b'Empty page!!' in e.output or (
+            e.output == b'' and e.returncode == 1
+        ):  # Not enough info for a skew angle - Tess 4 and 5 return different errors
             return 0.0
 
         raise SubprocessOutputError() from e

tests/test_preprocessing.py

@@ -40,6 +40,11 @@ def test_deskew(resources, outdir):
     assert -0.5 < skew_angle < 0.5, "Deskewing failed"
 
 
+def test_deskew_blank_page(resources, outpdf):
+    # Tesseract doesn't like blank pages - make sure we can get through
+    check_ocrmypdf(resources / 'blank.pdf', outpdf, '--deskew')
+
+
 @pytest.mark.xfail(reason="remove background disabled")
 def test_remove_background(resources, outdir):
     # Ensure the input image does not contain pure white/black