fix(xds): do not log 'no keys' as an error when an xds key has no data

[AlinsRan]

Description

_automatic_fetch logs every non-nil error returned by sync_data, including the "no keys" sentinel that config.new returns when an xds key simply has no data yet:

if err then
    log.error("failed to fetch data from xds ", err, ", ", key)
end

The periodic fetch loop a few lines up already treats "no keys" as non-error (err ~= "no keys"). This makes _automatic_fetch consistent with it, so a config provider watching an empty xds key no longer spams error.log at startup.

Which issue(s) this PR fixes

Removes spurious failed to fetch data from xds no keys error-log lines for xds keys that are legitimately empty.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change — this only changes whether an existing non-error condition is logged as an error; it mirrors the err ~= "no keys" filter the periodic loop already applies, and the xds test harness needs a preloaded .so, so there is no lightweight unit for the empty-key path
• I have updated the documentation accordingly