Updated MexcAuthenticationProvider to skip header creation for unauthenticated requests

[nils2525]

The following test failed on the second GetKLinesAsync() call:

var client = new MexcRestClient(null, new TestLoggerFactory(), Options.Create<MexcRestOptions>(new()));
// Success
var successResponse = await client.SpotApi.ExchangeData.GetKlinesAsync("BTCUSDT", Mexc.Net.Enums.KlineInterval.OneMinute, limit: 2);

client.SetApiCredentials(new("x", "y"));
// Failed
var failedResponse= await client.SpotApi.ExchangeData.GetKlinesAsync("BTCUSDT", Mexc.Net.Enums.KlineInterval.OneMinute, limit: 2);

Error Message: Error response: [ServerError] 700004: Mandatory parameter 'signature' was not sent, was empty/null, or malformed. , 99 bytes received in 256ms

I noticed that the X-MEXC-APIKEY header was set even though authentication was not required for the request. My change fixes the error. I hope that there was no specific reason that the key was set before the auth check.