Merge #12729: Get rid of ambiguous OutputType::NONE value

1e46d8a Get rid of ambiguous OutputType::NONE value (Russell Yanofsky)

Pull request description:

  Based on suggestion by @sipa https://github.com/bitcoin/bitcoin/pull/12119#issuecomment-357982763

  After #12119, the NONE output type was overloaded to refer to either an output type that couldn't be parsed, or to an automatic change output mode.  This change drops the NONE enum and uses a simple bool to indicate parse failure, and a new CHANGE_AUTO enum to refer the change output type.

  This change is almost a pure refactoring except it makes RPCs reject empty string ("") address types instead of treating them like they were unset. This simplifies the parsing code a little bit and could prevent RPC usage mistakes. It's noted in the release notes.

  Follows up #12408 by @MarcoFalke

  Followups for future PRs:

  - [ ] Add explicit support for specifying "auto" in `ParseOutputType` as suggested by promag and sipa: https://github.com/bitcoin/bitcoin/pull/12729#issuecomment-374799567 and https://github.com/bitcoin/bitcoin/pull/12729#discussion_r175969481
  - [ ] Add wallet `AddressChangeType` method to complement `TransactionChangeType`:  https://github.com/bitcoin/bitcoin/pull/12729#discussion_r175969618.

Tree-SHA512: 8b08b272bcb177a0a9e556dcd965840a7fe601ef83ca97938b879c9b1a33b5b3f96939e1bceef11ba7c644ac21bfd6c1dbc6ca715cd1da4ace50475240e4ee48

doc/release-notes.md

@@ -104,6 +104,11 @@ Low-level RPC changes
   now the empty string `""` instead of `"wallet.dat"`. If bitcoin is started
   with any `-wallet=<path>` options, there is no change in behavior, and the
   name of any wallet is just its `<path>` string.
+- Passing an empty string (`""`) as the `address_type` parameter to
+  `getnewaddress`, `getrawchangeaddress`, `addmultisigaddress`,
+  `fundrawtransaction` RPCs is now an error. Previously, this would fall back
+  to using the default address type. It is still possible to pass null or leave
+  the parameter unset to use the default address type.
 
 - Bare multisig outputs to our keys are no longer automatically treated as
   incoming payments. As this feature was only available for multisig outputs for