Don't try to run jbig2 when not available

src/ocrmypdf/_optimize.py

@@ -133,7 +133,7 @@ def extract_image(*, doc, pike, root, log, image, xref, jbig2s,
         return False
     filtdp = filtdps[0]
 
-    if bpc == 1 and filtdp[0] != '/JBIG2Decode':
+    if bpc == 1 and filtdp[0] != '/JBIG2Decode' and jbig2enc.available():
         if filtdp[0] == '/CCITTFaxDecode':
             data = image.read_raw_bytes()
             try:

src/ocrmypdf/exec/jbig2enc.py

@@ -22,7 +22,7 @@ import os
 import shutil
 
 from . import get_version
-from ..exceptions import ExitCode
+from ..exceptions import ExitCode, MissingDependencyError
 
 
 @lru_cache(maxsize=1)
@@ -30,6 +30,15 @@ def version():
     return get_version('jbig2enc', regex=r'jbig2enc (\d+(\.\d+)*).*')
 
 
+@lru_cache(maxsize=1)
+def available():
+    try:
+        version()
+    except MissingDependencyError:
+        return False
+    return True
+
+
 def convert_group(*, cwd, infiles, out_prefix):
     args = [
         'jbig2',