Improve util/join-maps to remove nils 

(join-maps {"c1" 2} {"c2" 1} {"c3" 3})
current, {c1 (2 nil nil), c2 (nil 1 nil), c3 (nil nil 3)}
my fix, {c1 (2), c2 (1), c3 (3)}

I also tryed other way, but the sequence of map items will be change.
```Clojure
(defn join-maps [& maps]
"({:a 1 :b 2 :c 3} {:a 2 :c 4})->{:b (2), :c (3 4), :a (1 2)}" 
  ( map-val seq
    (reduce (fn [m map] 
            (reduce (fn [m [k,v]] (assoc m k (conj (get m k []) v))) m map))
    {} maps))
)
```
So I just remove the nils on oringal logic finally