text: Title: datamodel-codegen produces class with missing body parts
  Body: **Describe the bug**
  
  Within the OSCAL schema is:
  
  ```
  "EmailAddressDatatype" : 
     { "description" : "An email address string formatted according to RFC 6531.",
      "allOf" : 
      [ 
       { "$ref" : "#/definitions/StringDatatype" },
       
       { "type" : "string",
        "format" : "email",
        "pattern" : "^.+@.+$" } ] },
  ```
  
  An **empty body** is produced by datamodel-codegen:
  
  ```
  class EmailAddressDatatype(OscalBaseModel):
      """
      An email address string formatted according to RFC 6531.
      """
  ```
  
  **To Reproduce**
  1. download OSCAL schemas from here [https://github.com/usnistgov/OSCAL/releases/tag/v1.1.2](https://github.com/usnistgov/OSCAL/releases/tag/v1.1.2)
  2. run datamodel-codegen (shown below)
  
  Example schema (snippet):
  ```json
  "EmailAddressDatatype" : 
     { "description" : "An email address string formatted according to RFC 6531.",
      "allOf" : 
      [ 
       { "$ref" : "#/definitions/StringDatatype" },
       
       { "type" : "string",
        "format" : "email",
        "pattern" : "^.+@.+$" } ] },
  ```
  Follow link above for full schema.
  
  
  Used commandline:
  ```
  datamodel-codegen --disable-timestamp --disable-appending-item-suffix --use-schema-description --input-file-type jsonschema --input release-1.1.2-schemas/oscal_assessment-plan_schema.json --base-class trestle.core.base_model.OscalBaseModel --output trestle/oscal/tmp/assessment_plan.py
  ```
  
  **Expected behavior**
  The generated code body should not be empty, but rather contain that which is found in the json schema.
  
  **Version:**
   - OS: Red Hat Enterprise Linux release 8.9 (Ootpa)
   - Python version: 3.9.9
   - datamodel-code-generator version: 0.25.5
  
  **Additional context**
  NA
  