refactor: Block unsafe fs::path std::string conversion calls

There is no change in behavior. This just helps prepare for the
transition from boost::filesystem to std::filesystem by avoiding calls
to methods which will be unsafe after the transaction to std::filesystem
to due lack of a boost::filesystem::path::imbue equivalent and inability
to set a predictable locale.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
Co-authored-by: Kiminuo <kiminuo@protonmail.com>
Co-authored-by: MarcoFalke <falke.marco@gmail.com>

src/wallet/walletutil.cpp

@@ -12,7 +12,7 @@ fs::path GetWalletDir()
     fs::path path;
 
     if (gArgs.IsArgSet("-walletdir")) {
-        path = gArgs.GetArg("-walletdir", "");
+        path = fs::PathFromString(gArgs.GetArg("-walletdir", ""));
         if (!fs::is_directory(path)) {
             // If the path specified doesn't exist, we return the deliberately
             // invalid empty string.