Fix error when supplying all config values from env variables

If there was no config file and all config values were loaded from env
variables, the userConfig object was never assigned a value and would
throw an error when loading env variables.

src/config.js

@@ -12,7 +12,7 @@
 const fs = require('fs');
 const path = require('path');
 
-let userConfig;
+let userConfig = {};
 
 // Config files to search for, in priority order
 const configFiles = [