fix logic of autohint parameter

an empty string (autohint='') means run ttfautohint with default options.
autohint=None means don't run ttfautohint.

Lib/fontmake/font_project.py

@@ -546,11 +546,17 @@ def save_otfs(
                     font["GSUB"] = gsub_src["GSUB"]
 
             # Decide on autohinting and its parameters
-            autohint_thisfont = ttf and (
-                autohint or ufo.lib.get(AUTOHINTING_PARAMETERS)
+            autohint_thisfont = (
+                (
+                    autohint
+                    if autohint is not None
+                    else ufo.lib.get(AUTOHINTING_PARAMETERS)
+                )
+                if ttf
+                else None
             )
 
-            if autohint_thisfont:
+            if autohint_thisfont is not None:
                 # if we are autohinting, we save the unhinted font to a
                 # temporary path, and the hinted one to the final destination
                 fd, otf_path = tempfile.mkstemp("." + ext)
@@ -576,7 +582,7 @@ def save_otfs(
             ):
                 self.subset_otf_from_ufo(otf_path, ufo)
 
-            if not autohint_thisfont:
+            if autohint_thisfont is None:
                 continue
 
             if output_path is not None:

tests/test_main.py

@@ -719,7 +719,15 @@ def test_main_with_filter(data_dir, tmp_path):
     platform.python_implementation() == "PyPy",
     reason="ttfautohint-py doesn't work with pypy",
 )
-def test_autohinting(data_dir, tmp_path):
+@pytest.mark.parametrize(
+    "autohint_options",
+    [
+        (),
+        ("-a",),
+        ("--autohint", "-D latn"),
+    ]
+)
+def test_autohinting(data_dir, tmp_path, autohint_options):
     shutil.copytree(data_dir / "AutohintingTest", tmp_path / "sources")
 
     fontmake.__main__.main(
@@ -731,6 +739,7 @@ def test_autohinting(data_dir, tmp_path):
             "-i",
             "--output-dir",
             str(tmp_path),
+            *autohint_options,
         ]
     )