`get_descriptor_for_keychain(KeychainKind::Internal)` should return `None` if there is no internal keychain

[w0xlt]

The code below returns the external descriptor.

I understand that external descriptor will be used as (internal) change descriptor if it does not exist, but I think it would be better and less ambiguous if the function returned None or Error when explicitly calling the internal descriptor.

This would also make a better distinction between external and internal descriptors and be more in line with the wallet structure.

let wallet = Wallet::new_offline(
        "<desc>",
        None,
        network,
        MemoryDatabase::default(),
)
.unwrap();

let desc = wallet.get_descriptor_for_keychain(KeychainKind::Internal);

println!("desc: {}", desc.to_string());

[LLFourn]

Or maybe just has_descriptor_for_keychain returning a bool.

[w0xlt]

Good suggestion.
I opened #528 to add this function.

[LLFourn]

As mentioned in #528 you can do this with Wallet::public_descriptor and checking if the return value is is_none().