filesystem: Use std::string's empty() function instead of comparing against a literal

This is simply a basic value check as opposed to potentially doing
string based operations (unlikely, but still, avoiding it is free).

src/core/hle/service/filesystem/filesystem.cpp

@@ -23,7 +23,7 @@ constexpr u64 EMULATED_SD_REPORTED_SIZE = 32000000000;
 
 static FileSys::VirtualDir GetDirectoryRelativeWrapped(FileSys::VirtualDir base,
                                                        const std::string& dir_name) {
-    if (dir_name == "." || dir_name == "" || dir_name == "/" || dir_name == "\\")
+    if (dir_name.empty() || dir_name == "." || dir_name == "/" || dir_name == "\\")
         return base;
 
     return base->GetDirectoryRelative(dir_name);