Use CheckDataDirOption() for code uniformity

All other clients and tools use CheckDataDirOption() rather
fs::is_directory(GetDataDir(false)) for the first datadir check.

src/bitcoin-wallet.cpp

@@ -55,7 +55,7 @@ static bool WalletAppInit(int argc, char* argv[])
     // check for printtoconsole, allow -debug
     LogInstance().m_print_to_console = gArgs.GetBoolArg("-printtoconsole", gArgs.GetBoolArg("-debug", false));
 
-    if (!fs::is_directory(GetDataDir(false))) {
+    if (!CheckDataDirOption()) {
         tfm::format(std::cerr, "Error: Specified data directory \"%s\" does not exist.\n", gArgs.GetArg("-datadir", "").c_str());
         return false;
     }