fix(wallet): autobackup fixes (#5269)

## Issue being fixed or feature implemented
pls see individual commits

fixes an issue (reported by @strophy recently) where mixing wouldn't
start in a fresh new wallet

not 100% sure but
[99867eb](99867eb769)
might also fix #5350 reported by @splawik21 so this could also be a v19
backport candidate

## What was done?


## How Has This Been Tested?
mixing on testnet

## Breaking Changes


## Checklist:
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation

**For repository code-owners and collaborators only**
- [x] I have assigned this pull request to a milestone

src/wallet/wallet.cpp

@@ -324,6 +324,12 @@ std::shared_ptr<CWallet> CreateWallet(interfaces::Chain& chain, const std::strin
             wallet->m_spk_man->NewKeyPool();
             // end TODO
 
+            // backup the wallet we just encrypted
+            if (!wallet->AutoBackupWallet("", error, warnings) && !error.original.empty()) {
+                status = DatabaseStatus::FAILED_ENCRYPT;
+                return nullptr;
+            }
+
             // Relock the wallet
             wallet->Lock();
         }
@@ -4637,11 +4643,16 @@ bool CWallet::AutoBackupWallet(const fs::path& wallet_path, bilingual_str& error
         strWalletName = "wallet.dat";
     }
 
-    if (!IsBDBFile(BDBDataFile(wallet_path))) {
+    if (!wallet_path.empty() && !IsBDBFile(BDBDataFile(wallet_path))) {
         WalletLogPrintf("Automatic wallet backups are currently only supported with Berkeley DB!\n");
         return false;
     }
 
+    if (IsWalletFlagSet(WALLET_FLAG_BLANK_WALLET)) {
+        WalletLogPrintf("Wallet is blank, won't create new backup for it!\n");
+        return false;
+    }
+
     if (nWalletBackups <= 0) {
         WalletLogPrintf("Automatic wallet backups are disabled!\n");
         return false;