fix: d4dc4e2-introduced error (#100)

* adds helpful error message when files is overridden or empty

* fix: d4dc4e2-introduced error

The prior commit to this branch d4dc4e2 introduced an error
in the util/config.js file where it checks for the length of
the content.files property, whether that property exists in
the config or not.

src/util/config-file.js

@@ -24,7 +24,8 @@ function writeConfig(configPath, content) {
   if (!content.projectName) {
     throw new Error(`Error! Project name is not set in ${configPath}`)
   }
-  if (!content.files.length) {
+  
+  if (content.files && !content.files.length) {
     throw new Error(
       `Error! Project files was overridden and is empty in ${configPath}`,
     )