Fix catching too broad a set of fields

Only need to expand type fields, not everything.

openapi2jsonschema/util.py

@@ -61,12 +61,12 @@ def allow_null_optional_fields(data, parent=None, grand_parent=None, key=None):
                 for x in v:
                     new_v.append(allow_null_optional_fields(x, v, parent, k))
             elif isinstance(v, str):
-                is_null = k == "type" and v == "null"
+                is_non_null_type = k == "type" and v != "null"
                 has_required_fields = grand_parent and "required" in grand_parent
                 is_required_field = (
                     has_required_fields and key in grand_parent["required"]
                 )
-                if not is_null and not is_required_field:
+                if is_non_null_type and not is_required_field:
                     new_v = [v, "null"]
             new[k] = new_v
         return new