Gate use of custom directories behind a variable (#6157)

previous changes had forced every single user to use custom
directories for NAND and SDMC. Those paths were saved to the
config file and would interact badly with portable builds.

src/common/file_util.cpp

@@ -774,6 +774,9 @@ const std::string& GetDefaultUserPath(UserPath path) {
 }
 
 const void UpdateUserPath(UserPath path, const std::string& filename) {
+    if (filename.empty()) {
+        return;
+    }
     if (!FileUtil::IsDirectory(filename)) {
         LOG_ERROR(Common_Filesystem, "Path is not a directory. UserPath: {}  filename: {}", path,
                   filename);