fix: skip format data.head if not exist while loading toml config (#707)

Author: hsupu

lib/prepare/loadConfig.js

@@ -41,11 +41,13 @@ function parseConfig (file) {
     // reformat to match config since TOML does not allow different data type
     // https://github.com/toml-lang/toml#array
     const format = []
-    Object.keys(data.head).forEach(meta => {
-      data.head[meta].forEach(values => {
-        format.push([meta, values])
+    if (data.head) {
+      Object.keys(data.head).forEach(meta => {
+        data.head[meta].forEach(values => {
+          format.push([meta, values])
+        })
       })
-    })
+    }
     data.head = format
     break
   }