command: prevent server panic on graceful shutdown

[chrisroberts]

Description

When performing a graceful shutdown the client drain configuration
is checked for a deadline which is appended to the timeout. When
running as a server the client will not be set. Attempting to get
the drain deadline will result in a panic. This checks for the
client being available prior to fetching the deadline value.

Testing & Reproduction steps

Links

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[tgross]

LGTM!