Update mutation without columns

In Hasura 1.3.2 we configured a update mutation for a table with only one column with a preset. It's a way for the client to tell us everything is finished. It's just possible to change the state to 8.


The mutation with an empty set:
```
mutation {
update_insp_inspections(where: {id: {_eq: "b84ddb1b-96d8-45cc-a195-6c2a125ed336"}}, _set: {}) {
affected_rows
}
}
```

Since we upgraded from 1.3.2 to 2.0.6 it doesn't work anymore. The mutation doesn't appear in the schema.
```
{
"errors": [
{
"extensions": {
"path": "$.selectionSet.update_insp_inspections",
"code": "validation-failed"
},
"message": "field \"update_insp_inspections\" not found in type: 'mutation_root'"
}
]
}
```

Now we need to add a dummy column with update permission without preset. Then the mutations appears again in schema.

Is this change intended or is it a bug?

Reproduce the reported update_insp_inspections mutation on Hasura 2.0.6, using the empty _set object and the update permission with a preset described in the issue. Compare the generated mutation_root schema with 1.3.2 and determine whether the missing mutation is intended; done means the behavior is clarified or corrected with coverage for this permission configuration.

Written by the indexing model from the issue text.