Describe the bug
The following

enum TargetEnum
{
    Bar,
}

class Source
{
    string Quux { get; set; }
}

class Target
{
    string Quux { get; set; }
    TargetEnum? TheTarget { get; set; }
}

[MapValue(nameof(Target.TheTarget), TargetEnum.Bar))
static partial Target Map(this Source source);

incorrectly yields

RMG077: Cannot assign constant value TargetEnum.Bar of type TargetEnum to Target.TheTarget of type TargetEnum?

but should result in no error and valid generated code, because a non-null value of a type is always assignable to a nullable value of the same type.

The below cannot be used as a workaround, as typecasts are not permitted in attributes.

[MapValue(nameof(Target.TheTarget), (TargetEnum?)TargetEnum.Bar))

The below is a valid workaround, but undesirable.

[MapValue(nameof(Target.TheTarget), Use = nameof(GetNullableTargetEnumValue)))
static partial Target Map(this Source source);

static TargetEnum? GetNullableTargetEnumValue() => TargetEnum.Bar;

Environment (please complete the following information):

• Mapperly Version: 4.0.0
• Nullable reference types: enabled
• .NET Version: 8.0.403
• Target Framework: .net8.0
• Compiler Version: 4.11.0-3.24460.3 (5649376e)
• C# Language Version: 12.0
• IDE: VS2022.17.11.5
• OS: Windows 11 Pro 23H2 (22631.4169)

Additional context
#631