NullPointerException Thrown when Both `properties` and `allOf` are Defined in a `schema` Object

Description

Not sure if it's my spec's issue but a null pointer exception was thrown.

Swagger-codegen version

Latest code in 3.0.0 branch

Swagger declaration file content or url

openapi: 3.0.1
info:
  version: 4.176.0
  title: Linode API
servers:
- url: https://api.linode.com/v4
paths:
  /networking/firewalls/{firewallId}/rules:
    parameters:
      - name: firewallId
        in: path
        required: true
        schema:
          type: integer
    put:
      tags:
      - Networking
      summary: Firewall Rules Update
      operationId: updateFirewallRules
      requestBody:
        content:
          application/json:
            schema:
              allOf:
              - $ref: '#/components/schemas/Firewall/properties/rules'
              type: object
              properties:
                inbound:
                  type: array
                  items:
                    $ref: '#/components/schemas/FirewallRuleConfig'
                outbound:
                  type: array
                  items:
                    $ref: '#/components/schemas/FirewallRuleConfig'
      responses:
        '200':
          description: Firewall Rules updated successfully.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Firewall/properties/rules'
components:
  schemas:
    Firewall:
      type: object
      properties:
        id:
          type: integer
          readOnly: true
        rules:
          type: object
          properties:
            inbound:
              type: array
              items:
                $ref: '#/components/schemas/FirewallRuleConfig'
            outbound:
              type: array
              items:
                $ref: '#/components/schemas/FirewallRuleConfig'
    FirewallRuleConfig:
      type: object
      properties:
        protocol:
          type: string

Command line used for generation

java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -i ./openapi.yaml -l go -o ./swagger-client-go

Exception in thread "Thread-0" java.lang.RuntimeException: Could not process operation:
  Tag: class Tag {
    name: Networking
    description: null
    externalDocs: null
}
  Operation: updateFirewallRules
  Resource: put /networking/firewalls/{firewallId}/rules
  Exception: Cannot invoke "io.swagger.v3.oas.models.media.Schema.getProperties()" because "fileTypeSchema" is null
        at io.swagger.codegen.v3.DefaultGenerator.processOperation(DefaultGenerator.java:989)
        at io.swagger.codegen.v3.DefaultGenerator.processPaths(DefaultGenerator.java:891)
        at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:484)
        at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:796)
        at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:388)
        at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.media.Schema.getProperties()" because "fileTypeSchema" is null
        at io.swagger.codegen.v3.generators.DefaultCodegenConfig.isFileTypeSchema(DefaultCodegenConfig.java:4443)
        at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromResponse(DefaultCodegenConfig.java:2349)
        at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromOperation(DefaultCodegenConfig.java:2042)
        at io.swagger.codegen.v3.generators.go.GoClientCodegen.fromOperation(GoClientCodegen.java:118)
        at io.swagger.codegen.v3.DefaultGenerator.processOperation(DefaultGenerator.java:965)
        ... 5 more

Start with the supplied OpenAPI declaration and generation command, then trace DefaultGenerator.processOperation through DefaultCodegenConfig.fromResponse and isFileTypeSchema, with GoClientCodegen.fromOperation as the caller shown in the stack trace. Done means the reported schema no longer produces a NullPointerException during generation; the payload does not name a test file.