7687f7873 [wallet] Support creating a blank wallet (Andrew Chow)
Pull request description:
Alternative (kind of) to #14938
This PR adds a `blank` parameter to the `createwallet` RPC to create a wallet that has no private keys initially. `sethdseed` can then be used to make a clean wallet with a custom seed. `encryptwallet` can also be used to make a wallet that is born encrypted.
Instead of changing the version number as done in #14938, a wallet flag is used to indicate that the wallet should be blank. This flag is set at creation, and then unset when the wallet is no longer blank. A wallet becomes non-blank when a HD seed is set or anything is imported. The main change to create a blank wallet is primarily taken from #14938.
Also with this, the term "blank wallet" is used instead of "empty wallet" to avoid confusion with wallets that have balance which would also be referred to as "empty".
This is built on top of #15225 in order to fix GUI issues.
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