cros lint: Do not error out if no files provided.
cros lint: Do not error out if no files provided.

The "cros lint" command is used as a repo upload hook, and in some cases
there are no files provided for a commit (if the commit only deletes
files).  This change makes "cros lint" accept a lack of arguments
without complaint, only issuing a warning for any human looking at the
output.

BUG=chromium:266549
TEST=`cros lint` does not fail, but issues warning
TEST=`cros lint scripts/cbuildbot.py` still runs lint
TEST=Creating a CL that deletes a file after this one and seeing if repo
will upload it.

Change-Id: Ib5aa5d5b75c881fa664e680a802d61de64245053
Reviewed-on: https://chromium-review.googlesource.com/182937
Tested-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite