fix: allow to pass null and undefined as httpProxy

[Noobgam]

Very minor (but a bit annoying) thing.

    this.client = new SteamUser({
           httpProxy: config.proxyConfiguration
        });

fails because it renders http:// as a url to proxy

whoever maintains @types/steam-user (know it's not you), says nulls should work, but they also don't.
That patch should do the trick

[DoctorMcKay]

What is the value of config.proxyConfiguration in your example?

[Noobgam]

Nevermind, I'm delusional. I passed an empty string. Undefined works as well, it won't pass the typeof undefined == 'string' part.

What is the value of config.proxyConfiguration in your example?

But I think the patch could still be useful to someone, because the errors thrown by steamuser are barely visible in logs.
You get the debug of sorts (I'm logging them with pino)

GetCMListForConnect error: Invalid URL"
Waiting for the debugger to disconnect...
node:internal/url:818
      href = bindingUrl.parse(input, base, true);
                        ^

TypeError: Invalid URL
    at new URL (node:internal/url:818:25)
    at agent.createConnection (\code\node_modules\@doctormckay\stdlib\lib\http\proxyagent.js:20:19)
    at Agent.createSocket (node:_http_agent:324:26)
    at Agent.addRequest (node:_http_agent:276:10)
    at new ClientRequest (node:_http_client:341:16)
    at request (node:https:381:10)
    at \code\node_modules\@doctormckay\stdlib\lib\http\client\HttpClient.js:69:23
    at \code\node_modules\@doctormckay\stdlib\lib\promises\timeoutPromise.js:23:34
    at new Promise (<anonymous>)
    at timeoutPromise (\code\node_modules\@doctormckay\stdlib\lib\promises\timeoutPromise.js:13:12)
Emitted 'error' event on SteamUser instance at:
    at SteamUser._doConnection (\code\node_modules\steam-user\components\09-logon.js:314:10) {

Which doesn't exactly tell much.
But feel free to close the PR if you think this is intended behavior