fix: Fix nil deref in config set command

Currently if the user sets --auth.method without --auth.header, if
defaultAuther isn't defined then there is a nil deref.

Check for nil directly so we can give a helpeful error message instead.

cmd/config.go

@@ -99,7 +99,7 @@ func getProxyAuth(flags *pflag.FlagSet, defaultAuther map[string]interface{}) (a
 		return nil, err
 	}
 
-	if header == "" {
+	if header == ""  && defaultAuther != nil{
 		header = defaultAuther["header"].(string)
 	}