fix: return nil Bytes() if the Cid in undef.

[Jorropo]

No description provided.

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[Stebalien]

LGTM, but could you document this before merging?

[github-actions]

Suggested version: v0.3.0
Comparing to: v0.2.0 (diff)

Changes in go.mod file(s):

(empty)

gorelease says:

# github.com/ipfs/go-cid
## compatible changes
MustParse: added

# summary
Suggested version: v0.3.0

gocompat says:

(empty)