[MINOR] refactor to check for null peer

[macfarla]

PR description

Refactor of private method
Was passing peer.getId() as a param in both instance where it was called

• which is unnecessary since the peer is passed as a param
• and the null check is done within registerDisconnect
• added an extra trace log line since it may be pathological that this method is called with a null peer

Fixed Issue(s)

Fixes #6840

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[gfukushima]

lgtm