ob-clojure should not tangle with inserting (ns ..) line when no :ns specified.

[stardiviner <numbchild@gmail.com>]

The ob-clojure tangle will insert ~(ns ..)~ from babel header argument :ns or use defualt ~(ns user)~.

   #+begin_src clojure :eval no :ns "" :tangle "data/code/xunfei-clj-demo/project.clj" :results link :file "data/code/xunfei-clj-demo/project.clj"
   (defproject xunfei-clj-demo "0.1.0-SNAPSHOT"
     :description "xunfei-clj demo"
     :url "http://example.com/FIXME"
     :license {:name "Eclipse Public License"
               :url  "http://www.eclipse.org/legal/epl-v10.html"}
     :dependencies [[org.clojure/clojure "1.9.0"]
                    [xunfei-clj "0.1.4-SNAPSHOT"]]
     :resource-paths ["lib/Msc.jar"])
   #+end_src

In upper case, obviously I don't want insert the ~(ns ..)~ line of code.

I hope the ob-clojure header argument ~:ns~ should detect value like
"nil", "" (empty string) or something else. Or when ~:ns~ header
argument is not specified. So that the tangling will not auto insert
~(ns ..)~ line.

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[ stardiviner ]
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