Interfaces without a mutable member do not generate `Put` or `MergePatch` schema objects

[blakew]

Interfaces without a mutable member do not generate a Put or a MergePatch component, but they do create the references to them, producing invalid spec.

Reproducible example:

    TargetOptions:
      description: |
        The export target configuration options. These vary by connection type.
      discriminator:
        propertyName: connection_type
      oneOf:
        - $interface: "TargetOptionsBigQuery#SameAsInterface"
        - $interface: "TargetOptionsHostedApi#SameAsInterface"
        - $interface: "TargetOptionsHostedCsv#SameAsInterface"
        - $interface: "TargetOptionsRedshift#SameAsInterface"
        - $interface: "TargetOptionsS3Csv#SameAsInterface"

...

    TargetOptionsHostedApi:
      description: "Hosted API publication target configuration options."
      members:
        connection_type:
          required: true
          initializable: true
          mutable: false
          schema:
            const: hosted_api
            type: string

this produces:

TargetOptionsMergePatch:
      oneOf:
        - $ref: "#/components/schemas/TargetOptionsBigQueryMergePatch"
        - $ref: "#/components/schemas/TargetOptionsHostedApiMergePatch"
        - $ref: "#/components/schemas/TargetOptionsHostedCsvMergePatch"
        - $ref: "#/components/schemas/TargetOptionsRedshiftMergePatch"
        - $ref: "#/components/schemas/TargetOptionsS3CsvMergePatch"
      discriminator:
        propertyName: connection_type

...

    TargetOptionsPut:
      oneOf:
        - $ref: "#/components/schemas/TargetOptionsBigQueryPut"
        - $ref: "#/components/schemas/TargetOptionsHostedApiPut"
        - $ref: "#/components/schemas/TargetOptionsHostedCsvPut"
        - $ref: "#/components/schemas/TargetOptionsRedshiftPut"
        - $ref: "#/components/schemas/TargetOptionsS3CsvPut"
      discriminator:
        propertyName: connection_type

However, neither TargetOptionsHostedApiPut nor TargetOptionsHostedApiMergePatch are generated.