Symfony version(s) affected: 5.1.7

Description
When no base uri is set, and the url does not have a scheme, then you get a rather difficult to debug warning:
Trying to access array offset on value of type null.
When the code continues after the waring (in production for example), an error is thrown with the following message:

Symfony\Component\HttpClient\Exception\TransportException: URL using bad/illegal format or missing URL for "//localhost:8080/".

How to reproduce

The following code should reproduce the error:

HttpClient::create()->request('GET', 'localhost:8080');

Possible Solution
Right now there is no check to see if the scheme is present, the code just assumes there is. A more detailed error
should be thrown, to let the user know.

The fix for the user is to change the url to http://localhost:8080, but this may not be obvious right away (at least it wasn't for me).