This CL fixes the check for empty elements in keys.cc. Previously we would
accidentally bail out of the fast path because the check would always fail.
As a consequence for-in loops that would initialize the enum-cache of an object
with own-only fast properties would never be optimized properly.

Review-Url: https://codereview.chromium.org/2638323002
Cr-Commit-Position: refs/heads/master@{#42454}