avoid segmentation fault on accessing un-initialized struct field

[wf001]

What’s this PR?

What’s changed?

By using phi, avoid segmentation fault on the situation:

(defschema Country {
                    :name :: string
                  })

(defschema Person {
                   :name :: string
                   :country :: Country})

(def main :: int
  (fn []
    (let [co :: Country {:name "deutsch"}
          person :: Person {:name "jimmy"}]
      (prn (get (get person :country) :name))
      )))

Anything else?