Fix a bug in theme_inject: TypeError null (theme-next#961)

Author: jiangtj

scripts/filters/default-injects.js

@@ -7,6 +7,10 @@ const points = require('./../injects-point');
 hexo.extend.filter.register('theme_inject', (injects) => {
   let filePath = hexo.theme.config.custom_file_path;
 
+  if (!filePath) {
+    return;
+  }
+
   points.views.forEach((key) => {
     if (filePath[key]) {
       injects[key].file('custom', filePath[key]);