Update behaviour for bindings to categoryContext when cat is non-existant

src/runtime/python/pypgf.c

@@ -1917,6 +1917,10 @@ PGF_categoryContext(PGFObject *self, PyObject *args)
         return NULL;
     }
 
+    if (hypos == NULL) {
+        Py_RETURN_NONE;
+    }
+
     PyObject *contexts = PyList_New(n_hypos);
     if (contexts == NULL) {
         return NULL;

src/runtime/python/test_suite.py

@@ -98,7 +98,7 @@ def test_categoryContext_3(PGF):
     assert tup[2] == pgf.readType("N")
 
 def test_categoryContext_4(PGF):
-    assert PGF.categoryContext("X") == []
+    assert PGF.categoryContext("X") == None
 
 def test_functionIsConstructor_1(PGF):
     assert PGF.functionIsConstructor("s") == True