A null DefaultValue with DefaultValueIsQuoted = true throws NullReferenceException mid-declaration and leaves void M} #209

What's wrong

When DefaultValueIsQuoted is set, TemplateBase.HasDefaultValue (CodeBlocker/Templates/TemplateBase.cs:41) is true whatever the value, so WriteDefaultValueTo (:108-119) calls TemplateRendering.QuoteStringLiteral(DefaultValue). That method dereferences value.Length at TemplateRendering.cs:144, so a null value throws a NullReferenceException. By then the declaration is already partly written.

An unquoted null DefaultValue is accepted and treated as "no initialiser". The quoted path is the only one that crashes.

Repro (HEAD 032f69a)

var m = new MethodTemplate { Name = "M", Type = "void", BodyFactory = ... };
m.Parameters.Add(new ParameterTemplate { Name = "s", Type = "string", DefaultValue = null!, DefaultValueIsQuoted = true });
c.Members.Add(m);
cb.AddClass(c);

NullReferenceException: Object reference not set to an instance of an object.
class C
{
	void M}

A field or property with a quoted null default fails the same way.

Why it matters

A generator that maps an optional string from data, for example DefaultValue = attribute?.Value, DefaultValueIsQuoted = true, gets a bare NullReferenceException and a corrupt half-written class. It gets neither "" nor an up-front refusal. #198/#199 fixed this same symptom for null parameters and members, and DefaultValue's docs already say an empty quoted value writes "".

Suggested fix / acceptance criteria

• Treat a null quoted DefaultValue as empty, so it writes = "", the same as string.Empty. Alternatively, refuse it in Validate before anything is written, as Refuse a null parameter, member or nested type before writing [patch] #199 does.
• Add tests covering a parameter, a field and a property with DefaultValue = null, DefaultValueIsQuoted = true.