[stellar][cli]can't send payment to a federation recipient who includes memo

## Description
When sending a payment to a federation recipient that includes a memo, I got the error: `federation recipient included its own memo, but send called with a memo`, even if I didn't specify a memo.

```
$ http get 'https://api.yushi.moe/stellar/federation?type=name&q=binance_deposit*yushi.moe'
{
"account_id": "GAHK7EEG2WWHVKDNT4CEQFZGKF2LGDSW2IVM4S5DP42RBW3K6BTODB4A",
"memo": "1008446094",
"memo_type": "text",
"stellar_address": "binance_deposit*yushi.moe"
}

$ keybase wallet send binance_deposit*yushi.moe 0.001
Send 0.001 XLM to binance_deposit*yushi.moe? (type 'YES' to confirm): YES
▶ ERROR federation recipient included its own memo, but send called with a memo
```

## Suggested fix
If no memo is specified, [`NewMemoFromStrings`](https://github.com/keybase/stellarnet/blob/aea14c9e62f0cd365672eaee9b142b102479c5b8/memo.go#L63-L70) returns a `NewMemoNone`, rather than nil.

https://github.com/keybase/client/blob/ba8a26e4f9d290f14d50644f4c7e5f7d6f2c0749/go/stellar/stellar.go#L664-L666
https://github.com/keybase/client/blob/ba8a26e4f9d290f14d50644f4c7e5f7d6f2c0749/go/stellar/stellar.go#L800-L802

Start with NewMemoFromStrings in memo.go and the referenced memo checks in go/stellar/stellar.go around lines 664-666 and 800-802. Trace the no-memo send path and verify that a federation recipient's memo can be used when the sender did not provide one, while an explicitly supplied memo still behaves correctly.

Written by the indexing model from the issue text.