ShellPkg/Shell - Fix ASSERT() when FvSimpleFileSystemDxe is used

ShellPkg/Shell - Fix ASSERT() when FvSimpleFileSystemDxe is used

When the FvSimpleFileSystemDxe module is included in a platform,
Simple File System Protocols are produced for firmware volumes(FV)
that do not have the same style device paths as file systems with
file names.  The ShellPkg has an assumption that the device path
contains device path nodes of type MEDIA_FILEPATH_DP and generates
an ASSERT() if any other device path nodes are encountered.  This
change removes the ASSERT() condition and instead returns NULL that
means EfiShellGetFilePathFromDevicePath() can not convert the
device path nodes that represent the file path to a Unicode string.

Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19228 6f19259b-4bc3-4df7-8a09-765794883524