Incorrect spec text for "Empty Merged Object Type" rule

[glen-84]

As a post-merge validation rule, it should look at the merged schema, not at the source schemas.

[PascalSenn]

The empty merged object type rule currenlty is:

- Let {types} be the set of all input object types in the composite schema.
- For each {type} in {types}:
  - Let {fields} be a set of all fields in {type}.
  - {fields} must not be empty.

I think this is fixed.

But also, for certain post validation rules its valid to access the source schemas. The post merge validation still has the context of all source schemas. Depending on the implementation, this meta information is stored on the execution schema (e.g. super graph document in federation or execution schema in fusion). Yet, the specification does not assume how you want to preserve this information. It would be completely valid for a implementation to not have an "execution schema" but keep each source schema around for execution too.