fix!: allow empty memos (NULL op returns)

[coolaj86]

BREAKING CHANGE

• empty strings are now considered valid memos
• null is now the placeholder value for objects that complete the output interface

   let outputs = [
-   { satoshis: 10000, memo: '', message: '', address: 'xxxx' },
+   { satoshis: 10000, memo: null, message: null, address: 'xxxx' },
   ];

Previously an output that looked like this would be considered a payment to 'xxx'

let outputs = [
    { satoshis: 10000, memo: '', message: '', address: 'xxxx' },
];

Between this and #74 (which allows burn memos), it will now throw an error:

throw new Error(`memo outputs must not have 'address' or 'pubKeyHash'`);

Also, it seems that there may have been a regression of #56, because this does the same that that says in the title.

Tested, working

https://trpc.digitalcash.dev/#?method=getrawtransaction&params=["79e822ae946a2babb141291c052da999d3987f0529cbb3cece02cc2377997a59",1]&submit