Remove nil values from filespec

When resource-path isn't specified write-jar blows up on the nil value in filespec giving the following exception:

Exception in thread "main" java.lang.IllegalArgumentException: No method in multimethod 'copy-to-jar' for dispatch value: null (NO_SOURCE_FILE:0)
	at clojure.lang.Compiler.eval(Compiler.java:5440)
	at clojure.lang.Compiler.eval(Compiler.java:5391)
	at clojure.core$eval.invoke(core.clj:2382)
	at clojure.main$eval_opt.invoke(main.clj:235)
	at clojure.main$initialize.invoke(main.clj:254)
	at clojure.main$script_opt.invoke(main.clj:270)
	at clojure.main$main.doInvoke(main.clj:354)
	at clojure.lang.RestFn.invoke(RestFn.java:457)
	at clojure.lang.Var.invoke(Var.java:377)
	at clojure.lang.AFn.applyToHelper(AFn.java:172)
	at clojure.lang.Var.applyTo(Var.java:482)
	at clojure.main.main(main.java:37)
Caused by: java.lang.IllegalArgumentException: No method in multimethod 'copy-to-jar' for dispatch value: null
	at clojure.lang.MultiFn.getFn(MultiFn.java:121)
	at clojure.lang.MultiFn.invoke(MultiFn.java:172)
	at leiningen.jar$write_jar.invoke(jar.clj:142)
	at leiningen.hadoop$jar.invoke(hadoop.clj:33)
	at leiningen.hadoop$hadoop.invoke(hadoop.clj:41)
	at clojure.lang.Var.invoke(Var.java:365)
	at clojure.lang.AFn.applyToHelper(AFn.java:161)
	at clojure.lang.Var.applyTo(Var.java:482)
	at clojure.core$apply.invoke(core.clj:542)
	at leiningen.core$apply_task.invoke(core.clj:259)
	at leiningen.core$_main.doInvoke(core.clj:325)
	at clojure.lang.RestFn.invoke(RestFn.java:410)
	at clojure.lang.AFn.applyToHelper(AFn.java:161)
	at clojure.lang.RestFn.applyTo(RestFn.java:132)
	at clojure.core$apply.invoke(core.clj:542)
	at leiningen.core$_main.invoke(core.clj:328)
	at user$eval42.invoke(NO_SOURCE_FILE:1)
	at clojure.lang.Compiler.eval(Compiler.java:5424)
	... 11 more