[fix] [export] Only include :resource-dirs when the user declares it

[kimo-k]

Problem: If there's no :resource-dirs in the user config, clein will export an invalid map. For instance:

(defn make-opts [opts]
  (let [lib     're-com/re-com
        version (make-version)]
    (assoc opts
           :lib lib
           :version version
           :scm {:url "https://github.com/day8/re-com"
                 :tag (str "v" version)}
           :main 're-com.core
           :basis (b/create-basis {:project "deps.edn"})
           :provided (b/create-basis {:project "deps.edn"
                                      :aliases [:provided]})
           :class-dir "target/classes"
           :jar-file (str (io/file "target" (format "%s-%s.jar" (name lib) version)))
           :uber-file (str (io/file "target" (format "%s-%s-standalone.jar" (name lib) version)))
           :src-dirs ["src"]
           :resource-dirs                             ;; <- PROBLEM
           :javac-opts nil
           :pom-data [[:licenses
                  [:license
                   [:name "MIT"]
                   [:url "https://opensource.org/licenses/MIT"]]]])))