[BUG][C#] anyOf generates non-compilable code (null-coalescing operator on enum and guid)

[PetervdHemel]

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?
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Description

Ever since updating to v7.4.0 which claims to 'fix anyof, oneof mixed primitive object parse error' I've been unable to generate, producing the following errors:

error CS0019: Operator '??' cannot be applied to operands of 
 type 'Guid' and '<throw expression>'

Which leads to this generated C# code:

public ParentId(Guid actualInstance)
{
        this.IsNullable = false;
        this.SchemaType= "anyOf";
        this.ActualInstance = actualInstance ?? throw new ArgumentException("Invalid instance found. Must not be null.");
}

error CS0019: Operator '??' cannot be applied to operands  
of type 'TaskStatusFilter' and '<throw expression>'

Where TaskStatusFilter is an enum, and leads to the following C# code:

public TaskStatus(TaskStatusFilter actualInstance)
{
        this.IsNullable = false;
        this.SchemaType= "anyOf";
        this.ActualInstance = actualInstance ?? throw new ArgumentException("Invalid instance found. Must not be null.");
}

Note that before v7.4.0 these exact issues did not exist, but I had issues with AnyOf instead, which is why I wanted to upgrade to solve this particular issue.

error CS0246: The type or namespace name 'AnyOf' could not be f 
ound (are you missing a using directive or an assembly reference?)

(related to a different part of the spec that uses 'AnyOf' including optional null value)

openapi-generator version

7.4.0

OpenAPI declaration file content or url

Example (shortened) openapi.json

Steps to reproduce

generate C# project with any specification file containing similar anyOf that are nullable

Related issues/PRs

#17986
#5442
#13384

Suggest a fix

The null-coalescing operator should not be applied to objects of type Guid or otherwise non-nullable types (eg custom types like TaskStatusFilter here)

[frekvent-szabolcs]

Same issue with 7.4.0 and with this specification (Directus default spec):
oas.json

[andyatshippo]

Same issue with 7.8.0 and this specification (Shippo public API)