Add default empty value for lvs.icv.equiv_files key

[sunjin-choi]

Background work

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• This issue is specific to Hammer, i.e., does not belong in a plugin repository instead
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Hammer version and plugin hashes

Minor issue with the updated ICV key in #879
@daniellovell

Other Setup

No response

Current Behavior

lvs.icv.equiv_files key may need to be added into the defaults.yml under lvs/icv with an empty value.

Expected Behavior

It is currently fine for users to add the empty value for this key in the user space, but easier this way for backward compatibility.

Probably can be incorporated into the next PR.

Other Information

No response