libefivar: Handle AcpiExp device path when optional para is not specified

AcpiExp text device path: AcpiExp(HID,CID,UIDSTR)
And according to UEFI spec, the CID parameter is optional
and has a default value of 0. But current implementation
miss to check following cases for the AcpiExp.
FromText: when text device is AcpiExp(HID,,UIDSTR)/AcpiExp(HID,0,UIDSTR)
ToText: when the CID is 0 in the node structure

This commit is to do the enhancement.

Upstream Bug: https://bugzilla.tianocore.org/show_bug.cgi?id=1243
Obtained from: https://github.com/tianocore/edk2/commit/a8b5750901faa63ff5570634851e648b8e335e5a
Pull Request: https://github.com/freebsd/freebsd-src/pull/581