Nullable required/init properties should use nullable coalescing initialization when mapping child.

[TimothyMakkison]

When mapping nested children of a nullable property with MapProperty, Mapperly will use a nullable coalescing assignment expression to ensure that the parent is initialized and to prevent that a NullReferenceException does not occur.

This does not occur if the source member is required/init. Perhaps a nullable coalescing assignment could be inserted into the initializer to prevent errors from occuring.

Example of normal behaviour

[Mapper]
public static partial class Mapper
{
    [MapProperty("Value", "Nested.Value")]
    public static partial B Map(A src);
}

public class A { public int Value { get; set; } }

public class B { public C? Nested { get; set; } }

public class C { public int Value { get; set; } }

// Generates
public static partial global::Riok.Mapperly.Sample.B Map(global::Riok.Mapperly.Sample.A src)
{
    var target = new global::Riok.Mapperly.Sample.B();
    target.Nested ??= new();
    target.Nested.Value = src.Value;
    return target;
}

Desired behaviour

[Mapper]
public static partial class Mapper
{
    [MapProperty("Value", "Nested.Value")]
    public static partial B Map(A src);
}

public class A { public int Value { get; set; } }

public class B { public C? Nested { get; init; } }

public class C { public int Value { get; set; } }

// Generates
public static partial global::Riok.Mapperly.Sample.B Map(global::Riok.Mapperly.Sample.A src)
{
    var target = new global::Riok.Mapperly.Sample.B()
    {
        Nested ??= new global::Riok.Mapperly.Sample.C();
    };
    target.Nested.Value = src.Value;
    return target;
}

Current generation

public static partial global::Riok.Mapperly.Sample.B Map(global::Riok.Mapperly.Sample.A src)
{
    var target = new global::Riok.Mapperly.Sample.B();
    return target;
}