Fix decompilation of record with missing base type

[andrewcrawley]

Problem

As described in #3020, the decompiler fails with a NullReferenceException when attempting to decompile a record type whose base type cannot be determined (e.g. because the base type is defined in another assembly that is not loaded).

Solution

This commit updates RecordDecompiler to null check the baseClass field before making use of it. With this fix, the null ref is avoided, but the generated code for the record will contain unnecessary [CompilerGenerated] members:

internal record RemovedSettingRegistration : SettingRegistration
{
	public RemovedSettingRegistration(SettingRegistration original)
		: base(original)
	{
	}

	[CompilerGenerated]
	public override string ToString() { ... }

	[CompilerGenerated]
	protected override bool PrintMembers(StringBuilder builder) { ... }

	[CompilerGenerated]
	public override int GetHashCode() { ... }

	[CompilerGenerated]
	public sealed override bool Equals(SettingRegistration? other) { ... }

	[CompilerGenerated]
	public virtual bool Equals(RemovedSettingRegistration? other) { ... }

	[CompilerGenerated]
	protected RemovedSettingRegistration(RemovedSettingRegistration original)
		: base((SettingRegistration)(object)original)
	{
	}
}