Added some tests for create_dir

include/spdlog/details/file_helper-inl.h

@@ -34,10 +34,8 @@ SPDLOG_INLINE void file_helper::open(const filename_t &fname, bool truncate)
 
     for (int tries = 0; tries < open_tries_; ++tries)
     {
-        if (!folder_name.empty())
-        {
-            os::create_dir(folder_name); // will not created if already exists
-        }
+        // will not created if already exists or empty.
+        os::create_dir(folder_name);
 
         if (!os::fopen_s(&fd_, fname, mode))
         {

include/spdlog/details/os-inl.h

@@ -478,7 +478,7 @@ SPDLOG_INLINE bool mkdir_(const filename_t &path)
 }
 
 // create the given directory - and all directories leading to it
-// return true on success
+// return true on success or if the directory already exists
 SPDLOG_INLINE bool create_dir(filename_t path)
 {
     if (path_exists(path))
@@ -486,6 +486,11 @@ SPDLOG_INLINE bool create_dir(filename_t path)
         return true;
     }
 
+    if(path.empty())
+    {
+        return false;
+    }
+
 #ifdef _WIN32
     // support forward slash in windows
     std::replace(path.begin(), path.end(), '/', folder_sep);