URL is not always set in on_aws_guzzle_request_stats

[rmccue]

We gather the URL from this line:

https://github.com/humanmade/aws-xray/blob/fb9c4b1ddf69a517991caceae16d169ed0ce5913/inc/namespace.php#L576

In some cases, no URL is getting logged. These also happen to be requests that (apparently) take no time at all.

e.g. trace 1-5e43fecd-65da2d68f62e3cb0a8b41abc; ping me for more details