fix None enum struct (#995)

Author: jklugherz

v03_pipeline/lib/reference_datasets/exac_test.py

@@ -52,3 +52,9 @@ def test_exac(self):
                     ),
                 ],
             )
+            self.assertEqual(
+                ht.globals.collect(),
+                [
+                    hl.Struct(version='1.1', enums=hl.Struct()),
+                ],
+            )

v03_pipeline/lib/reference_datasets/reference_dataset.py

@@ -90,7 +90,7 @@ def enums(self) -> dict | None:
     def enum_globals(self) -> hl.Struct:
         if self.enums:
             return hl.Struct(**self.enums)
-        return hl.missing(hl.tstruct(hl.tstr, hl.tarray(hl.tstr)))
+        return hl.Struct()
 
     @property
     def filter(  # noqa: A003