compute/metadata: use defaultClient when http.Client is nil
compute/metadata: use defaultClient when http.Client is nil

If the argument `c` of NewClient is nil, use the defaultClient.

Fixes #1794

Change-Id: I3e233df9ea30a7381fe5ae52e07b3fea8ddcd49d
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/52031
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Cody Oss <f11fa1e55ff5638510f5bf86ea76672a630250f9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go