fix: merge nil/empty + conj semantics to match Clojure

core-merge returned {} for (merge) and errored on nil args. Rewrite to follow
Clojure: (when (some identity maps) (reduce conj (or (first maps) {}) (rest maps))).

- (merge) and (merge nil nil) -> nil
- nil args elsewhere are no-ops: (merge {} nil) -> {}, (merge nil {:a 1}) -> {:a 1}
- later args follow conj semantics: a map merges its entries, a [k v]
  vector/map-entry adds that entry ((merge {} [:foo 1]) -> {:foo 1},
  (merge {} (first {:a 1})) -> {:a 1})
- collection keys preserved (first arg's phm type kept; entries via core-assoc
  which promotes struct->phm for collection keys)

spec: 9 new merge cases in maps-spec. merge.cljc 13/18/11 -> 29/11/2.
clojure-test-suite pass 1688->1704. jpm test green.

Remaining merge.cljc fails are the lenient-where-Clojure-throws class (atomic
args, >2-element vectors). Fixes jolt-dxx.

test/spec/maps-spec.janet

@@ -34,6 +34,15 @@
   ["dissoc many"            "{:a 1}"          "(dissoc {:a 1 :b 2 :c 3} :b :c)"]
   ["merge"                  "{:a 1, :b 2}"    "(merge {:a 1} {:b 2})"]
   ["merge overwrites"       "{:a 2}"          "(merge {:a 1} {:a 2})"]
+  ["merge lattermost wins"  "{:a 3}"          "(merge {:a 1} {:a 2} {:a 3})"]
+  ["merge no args -> nil"   "nil"             "(merge)"]
+  ["merge all nil -> nil"   "nil"             "(merge nil nil)"]
+  ["merge nil arg no-op"    "{:a 1}"          "(merge {:a 1} nil)"]
+  ["merge nil then map"     "{:a 1}"          "(merge nil {:a 1})"]
+  ["merge empty + nil"      "{}"              "(merge {} nil)"]
+  ["merge map-entry (conj)" "{:a 1}"          "(merge {} (first {:a 1}))"]
+  ["merge [k v] vector"     "{:foo 1}"        "(merge {} [:foo 1])"]
+  ["merge collection key"   "true"            "(= {[2 3] :foo} (merge {[2 3] :foo} nil {}))"]
   ["merge-with"             "{:a 3}"          "(merge-with + {:a 1} {:a 2})"]
   ["update"                 "{:a 2}"          "(update {:a 1} :a inc)"]
   ["update missing w/ fnil" "{:a 1}"          "(update {} :a (fnil inc 0))"]