[pgAdmin4][Patch]: Wrong listing of directories in File Manager in Windows Only.

Hi

FileManager: when Select File dialog is opened in query tool in Runtime Mode, it was listing wrong directory. instead it should list drives such as C:\, D:\, E:\ etc if no STORAGE_DIR path is set in config_local.py. This is a regression of RM 1937.

we do list drives, If platform is Windows and If self.dir is set to None, but self.dir is set to empty string inside __init__ constructor. This is the reason it wasn't listing drives.

I have taken care it in this patch and tested it for following platforms:

Windows,

Ubuntu and

Mac.

Please find attached patch and review.

Thanks,

Surinder Kumar