[csharp] Use default rather than null in ctor

[jimschubert]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

See #3608

This improves the defaults on constructor parameters by using defaults(type) rather than null for all inputs.

There seems to have been some weird usage of nullable types, possibly on parameters that aren't defined in the target swagger definition as nullable although I'm not 100% sure and didn't spend a lot of time investigating this.
Edit: I looked more closely and the nullable types seem to only occur when a primitive or enum is not marked as required. I had actually previously explained this behavior for properties, but wasn't thinking broadly enough when I made comment about constructor parameters above.

This PR changes constructor parameters in models to:

1. Default to a default if present
2. Set to default(TheType) if no default value is defined

Examples

Old Behavior: Dog

public Dog(string ClassName = null, string Color = null, string Breed = null)
{
    // to ensure "ClassName" is required (not null)
    if (ClassName == null)
    {
        throw new InvalidDataException("ClassName is a required property for Dog and cannot be null");
    }
    else
    {
        this.ClassName = ClassName;
    }
    // use default value if no "Color" provided
    if (Color == null)
    {
        this.Color = "red";
    }
    else
    {
        this.Color = Color;
    }
    this.Breed = Breed;
}

New Behavior: Dog

 public Dog(string ClassName = default(string), string Color = "red", string Breed = default(string))
{
    // to ensure "ClassName" is required (not null)
    if (ClassName == null)
    {
        throw new InvalidDataException("ClassName is a required property for Dog and cannot be null");
    }
    else
    {
        this.ClassName = ClassName;
    }
    // use default value if no "Color" provided
    if (Color == null)
    {
        this.Color = "red";
    }
    else
    {
        this.Color = Color;
    }
    this.Breed = Breed;
}

New Behavior: Other constructor examples

public Cat(string ClassName = default(string), 
            string Color = "red", 
            bool? Declawed = default(bool?)
) { }

public EnumArrays(JustSymbolEnum? JustSymbol = default(JustSymbolEnum?), 
            List<ArrayEnumEnum> ArrayEnum = default(List<ArrayEnumEnum>)
) { }

public Pet(long? Id = default(long?), 
           Category Category = default(Category), 
           string Name = default(string), 
           List<string> PhotoUrls = default(List<string>), 
           List<Tag> Tags = default(List<Tag>), 
           StatusEnum? Status = default(StatusEnum?)
) { }

I didn't include regenerated samples in this PR.

Question

If this looks good, should I go ahead and update aspnetcore and NancyFx generators to do the same for models?

[jimschubert]

@wing328 I'll try to get the same fix in place for aspnetcore and NancyFX within a day or so.