cmd/coordinator: don't dereference nil buildlet.Client
cmd/coordinator: don't dereference nil buildlet.Client

Fixes #50457

Change-Id: I54d5c5337588d1ce640a6fc9934802bc5020f454
Reviewed-on: https://go-review.googlesource.com/c/build/+/375754
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Reviewed-by: Alex Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Trust: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>

golang/build,golang/build,golang/build,golang/build