hiccup-seq containing nil causes NPE on conversion

[retrogradeorbit]

hiccup-seq conversion does not ignore nil elements like reagent.

$ bootleg -e '(convert-to [nil] :html)'
Exception in thread "main" java.lang.NullPointerException
	at bootleg.utils$convert_to.invokeStatic(utils.clj:274)
	at bootleg.utils$convert_to.invoke(utils.clj:271)
	at clojure.lang.AFn.applyToHelper(AFn.java:156)
	at clojure.lang.AFn.applyTo(AFn.java:144)
	at clojure.core$apply.invokeStatic(core.clj:665)
	at clojure.core$apply.invoke(core.clj:660)
	at sci.impl.interpreter$eval_call.invokeStatic(interpreter.cljc:286)
	at sci.impl.interpreter$eval_call.invoke(interpreter.cljc:250)
	at sci.impl.interpreter$interpret.invokeStatic(interpreter.cljc:302)
	at sci.impl.interpreter$interpret.invoke(interpreter.cljc:292)
	at sci.impl.interpreter$eval_do$fn__688.invoke(interpreter.cljc:64)
	at sci.impl.interpreter$eval_do.invokeStatic(interpreter.cljc:64)
	at sci.impl.interpreter$eval_do.invoke(interpreter.cljc:59)
	at sci.impl.interpreter$eval_edn_vals.invokeStatic(interpreter.cljc:355)
	at sci.impl.interpreter$eval_edn_vals.invoke(interpreter.cljc:354)
	at sci.impl.interpreter$eval_string.invokeStatic(interpreter.cljc:363)
	at sci.impl.interpreter$eval_string.invoke(interpreter.cljc:357)
	at sci.core$eval_string.invokeStatic(core.cljc:37)
	at sci.core$eval_string.invoke(core.cljc:5)
	at bootleg.hiccup$process_hiccup_data.invokeStatic(hiccup.clj:60)
	at bootleg.hiccup$process_hiccup_data.invoke(hiccup.clj:23)
	at bootleg.core$_main.invokeStatic(core.clj:67)
	at bootleg.core$_main.doInvoke(core.clj:52)
	at clojure.lang.RestFn.applyTo(RestFn.java:137)
	at bootleg.core.main(Unknown Source)