printSubgraphSchema fails if using a schema transformer and extending an external type

### Issue Description

If your subgraph has a schema that references an external type, e.g.
```gql
type SubgraphType {
id: ID!
external: ExternalType
}

extend type ExternalType @key(fields: "id") {
id: ID! @external
}
```

and you have a transformer applied to the schema, e.g.
```ts
const schemaText = fs.readFileSync(path.join(__dirname, 'schema.graphql')).toString()
const typeDefs = gql(schemaText)

let schema = buildSubgraphSchema([
{ typeDefs }
])

schema = basicSchemaTransformer(schema)
```

Then `printSubgraphSchema` will fail with the error
```
GraphQLError: Type ExternalType must define one or more fields.
at Object.err (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/error.ts:36:60)
at Validator.addError (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:138:45)
at Validator.validateObjectOrInterfaceType (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:168:12)
at Validator.validate (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:90:16)
at validateSchema (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:30:32)
at Schema.validate (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/definitions.ts:1606:42)
at Subgraph.validate (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/federation.ts:1766:19)
at buildSubgraph (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/federation.ts:1329:19)
at printSubgraphSchema (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/subgraph/src/printSubgraphSchema.ts:16:33)
at Object. (/Users/kgrant/git/print-subgraph-schema-error/src/print-schema.ts:4:32) {
message: '[] Type ExternalType must define one or more fields.',
path: undefined,
locations: undefined,
extensions: { code: 'INVALID_GRAPHQL' }
}
```

### Link to Reproduction

https://github.com/kal4l/print-subgraph-schema-error

### Reproduction Steps

* See reproduction repo

### Version

* `"@apollo/subgraph": "2.5.7"`

Start by running the linked reproduction repo with the schema transformer and the external type example. Inspect @apollo/subgraph/src/printSubgraphSchema.ts and the federation validation path shown in the stack trace. Done means printSubgraphSchema succeeds for this transformed schema and a regression test covers the case.

Written by the indexing model from the issue text.