[BUG] [C#] Validate method throws ArgumentNullException

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

The model code produced by the YAML file looks like this:

        /// <summary>
        /// To validate all properties of the instance
        /// </summary>
        /// <param name="validationContext">Validation context</param>
        /// <returns>Validation Result</returns>
        IEnumerable<System.ComponentModel.DataAnnotations.ValidationResult> IValidatableObject.Validate(ValidationContext validationContext)
        {
            // paymentId (string) pattern
            Regex regexpaymentId = new Regex(@"^[a-zA-Z0-9_-]{1,36}$", RegexOptions.CultureInvariant);
            if (false == regexpaymentId.Match(this.paymentId).Success)
            {
                yield return new System.ComponentModel.DataAnnotations.ValidationResult("Invalid value for paymentId, must match a pattern of " + regexpaymentId, new [] { "paymentId" });
            }

            // targetAmount (string) pattern
            Regex regextargetAmount = new Regex(@"^[0-9]{1,10}([.][0-9]{2})?$", RegexOptions.CultureInvariant);
            if (false == regextargetAmount.Match(this.targetAmount).Success) // EXCEPTION HERE
            {
                yield return new System.ComponentModel.DataAnnotations.ValidationResult("Invalid value for targetAmount, must match a pattern of " + regextargetAmount, new [] { "targetAmount" });
            }
[...]

The code above will throw

Value cannot be null. (Parameter 'input')</h1><br/>System.ArgumentNullException: Value cannot be null. (Parameter 'input')
   at System.Text.RegularExpressions.ThrowHelper.ThrowArgumentNullException(ExceptionArgument arg)
   at System.Text.RegularExpressions.Regex.Match(String input)
   at PblInitRequest.System.ComponentModel.DataAnnotations.IValidatableObject.Validate(ValidationContext validationContext)

The this.targetAmount is null (optional field) but it is pushed into a Regex#Match method anyway.

openapi-generator version

6.6.0
Additionaly checked with csharp codegen (former csharp-core) of
https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/7.0.0-SNAPSHOT/openapi-generator-cli-7.0.0-20230703.163913-170.jar

OpenAPI declaration file content or url

openapi: 3.0.0
paths:
  /api/tpp/payment-gate/init:
    post:
      tags:
        - PBL
      summary: xxSummary
      operationId: pblInitRequest
      description: xxDescription
      parameters: []
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/PblInitRequest'
      responses:
        '200':
          description: >-
            xxDescription
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/GenericInitResponseSuccess'

components:
  requestBodies:
    PblInitRequest:
      type: object
      required:
        - paymentId
      properties:
        paymentId:
          type: string
          pattern: /^[a-zA-Z0-9_-]{1,36}$/
          description: Unique payment id used to reference it all through the process.
        targetAmount:
          type: string
          pattern: /^[0-9]{1,10}([.][0-9]{2})?$/
          description: ------------ THIS FIELD IS OPTIONAL ------------ 

Generation Details

TPP_YAML = main/wwwroot/tpp/tpp.yaml
java -jar generated/openapi-generator-cli.jar generate \
-i $(TPP_YAML) \
-g csharp-netcore \
--additional-properties targetFramework=net6.0 \
--additional-properties packageName=${TPP_PKG} \
--additional-properties netCoreProjectFile=true \
--additional-properties library=generichost \
--additional-properties modelPropertyNaming=original \
--additional-properties dateTimeFormat=yyyy-MM-ddTHH:mm:ss.fff\'Z\' \
-o generated/tpp__temp

Related issues/PRs

Somehow related: #2760

Suggest a fix

Missing optional fields should not be validated against patterns (and possibly other formatting rules). Or maybe validation should always be protected with if (this.{{{name}}} != null && ....

Reproduce the csharp-netcore generation with the supplied YAML and inspect the generated model's IValidatableObject.Validate implementation. Done means an omitted optional targetAmount no longer causes ArgumentNullException during validation, while present patterned values remain validated.