[BUG][csharp-netcore] anyOf/oneOf generates non-compilable code (null-coalescing operator on enum)

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

AnyOf/OneOf with an enum will generate non-compilable C# code (using null coalescing operator on an enum):
For instance with a oneof parameter schema:

"schema": {
  "oneOf": [
	{
	"title": "firstEnum",
	"enum": [
		"one",
		"two"
	  ]
	},
	{
	"title": "secondEnum",
	"type": "integer",
	"enum": [
		"0",
		"1"
	]
	}
  ]
}

Incorrect code:

        /// <summary>
        /// Initializes a new instance of the <see cref="MyOperationAParameter" /> class
        /// with the <see cref="FirstEnum" /> class
        /// </summary>
        /// <param name="actualInstance">An instance of FirstEnum.</param>
        public MyOperationAParameter(FirstEnum actualInstance)
        {
            this.IsNullable = false;
            this.SchemaType= "oneOf";
            this.ActualInstance = actualInstance ?? throw new ArgumentException("Invalid instance found. Must not be null.");
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="MyOperationAParameter" /> class
        /// with the <see cref="SecondEnum" /> class
        /// </summary>
        /// <param name="actualInstance">An instance of SecondEnum.</param>
        public MyOperationAParameter(SecondEnum actualInstance)
        {
            this.IsNullable = false;
            this.SchemaType= "oneOf";
            this.ActualInstance = actualInstance ?? throw new ArgumentException("Invalid instance found. Must not be null.");
        }

Expected output:

        /// <summary>
        /// Initializes a new instance of the <see cref="MyOperationAParameter" /> class
        /// with the <see cref="FirstEnum" /> class
        /// </summary>
        /// <param name="actualInstance">An instance of FirstEnum.</param>
        public MyOperationAParameter(FirstEnum actualInstance)
        {
            this.IsNullable = false;
            this.SchemaType= "oneOf";
            this.ActualInstance = actualInstance;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="MyOperationAParameter" /> class
        /// with the <see cref="SecondEnum" /> class
        /// </summary>
        /// <param name="actualInstance">An instance of SecondEnum.</param>
        public MyOperationAParameter(SecondEnum actualInstance)
        {
            this.IsNullable = false;
            this.SchemaType= "oneOf";
            this.ActualInstance = actualInstance;
        }

openapi-generator version

6.0.1

OpenAPI declaration file content or url

anyOf/oneOf with an enum.
See in Description.

Steps to reproduce

generate csharp-netcore project with any specification file containing similar anyOf/oneOf.

Related issues/PRs

Suggest a fix

I'll reference this in fix PR.

Start by generating a csharp-netcore project from an OpenAPI specification containing the shown anyOf/oneOf enum schema, then inspect the generated constructors. Done means the generated C# code compiles and enum constructors no longer use the null-coalescing operator, matching the expected output.