MdePkg/UefiFileHandleLib: Tolerate more Root handle FileNames

MdePkg/UefiFileHandleLib: Tolerate more Root handle FileNames

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2295

The current implementation of the FileHandleGetFileName() function
assumes that the Root directory always has the FileName '\0'.
However, the only requirement the UEFI specification defines is that
a prepended '\\' must be supported to access files and folders
relative to the Root directory.
This patch removes this assumption and supports constructing valid
paths for any value of FileName for the Root Directory.

In practice, this fixes compatibility issues with File System drivers
that report '\\' as the FileName of the Root directory, which
currently is both generating an invalid path ("\\\\") and resulting
in an EFI_NOT_FOUND result from the CurrentHandle->Open() call.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Marvin Haeuser <mhaeuser@outlook.de>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>