prevent generating empty code section during vtable setup

Cython/Compiler/ModuleNode.py

@@ -1971,6 +1971,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             c_method_entries = [
                 entry for entry in type.scope.cfunc_entries
                 if entry.func_cname ]
+            if c_method_entries:
                 code.putln('#if PY_MAJOR_VERSION >= 3')
                 for meth_entry in c_method_entries:
                     cast = meth_entry.type.signature_cast_string()