Return empty string when END device path is passed in to align the original behavior in order to keep backward compatibility.

Signed-off-by: niruiyu
Reviewed-by: erictian

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12402 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/DevicePathDxe/DevicePathToText.c

@@ -1875,6 +1875,7 @@ ConvertDeviceNodeToText (
   //
   DumpNode (&Str, (VOID *) DeviceNode, DisplayOnly, AllowShortcuts);
 
+  ASSERT (Str.Str != NULL);
   return Str.Str;
 }
 
@@ -1959,5 +1960,9 @@ ConvertDevicePathToText (
     DevPathNode = NextDevicePathNode (DevPathNode);
   }
 
-  return Str.Str;
+  if (Str.Str == NULL) {
+    return AllocateZeroPool (sizeof (CHAR16));
+  } else {
+    return Str.Str;
+  }
 }