/api/metadata?includeTestLevel should work with no products

[gsnedders]

Currently:

% curl -i 'https://wpt.fyi/api/metadata?includeTestLevel'
HTTP/2 400 
date: Thu, 13 Mar 2025 20:14:43 GMT
content-type: text/plain; charset=utf-8
content-length: 33
server: nginx/1.26.3
strict-transport-security: max-age=31536000; preload
x-content-type-options: nosniff
via: 1.1 google

Missing required 'product' param

We shouldn't require a product param when includeTestLevel is included.