Validations for custom_type

[bendbennett]

Currently, the schema defines the following for custom_type:

        "schema_custom_type": {
          "type": "object",
          "properties": {
            "import": {
              "type": "string"
            },
            "type": {
              "type": "string"
            },
            "value_type": {
              "type": "string"
            }
          },
          "required": [
            "type",
            "value_type"
          ]
        },

The type and value_type properties should probably have validations on the schema to ensure that they are not empty strings.

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