Handle empty seq as empty string in query-string

example:

instead of

```clojure
(query-string {:nil nil
               :vec []
               :seq-empty '()})
;; => "nil=&&"
```

now

```clojure
(query-string {:nil nil
               :vec []
               :seq-empty '()})
;; => "nil=&vec=&seq-empty="
```

modules/reitit-core/src/reitit/impl.cljc

@@ -249,6 +249,10 @@
   (->> params
        (map (fn [[k v]]
               (if (or (sequential? v) (set? v))
+                (if (seq v)
                   (str/join "&" (map query-parameter (repeat k) v))
+                  ;; Empty seq results in single & character in the query string.
+                  ;; Handle as empty string to behave similarly as when the value is nil.
+                  (query-parameter k ""))
                 (query-parameter k v))))
        (str/join "&")))

test/cljc/reitit/impl_test.cljc

@@ -50,6 +50,8 @@
     {"a" "b"} "a=b"
     {:a 1} "a=1"
     {:a nil} "a="
+    {:a []} "a="
+    {:a '()} "a="
     {:a :b :c "d"} "a=b&c=d"
     {:a "b c"} "a=b+c"
     {:a ["b" "c"]} "a=b&a=c"