Cannot read property 'startsWith' of undefined. Fixes gh-75

Author: rwaldron

lib/config.js

@@ -43,6 +43,10 @@ class Config {
   }
 
   resolvePath(input) {
+    if (!input) {
+      return input;
+    }
+
     if (input.startsWith('~/')) {
       return path.join(os.homedir(), input.slice(2));
     } else if (input === '~') {