fix: conj 0-arg, conj onto nil (builds list), conj map into map

- (conj) -> []
- (conj nil x ...) builds a list (prepends): (conj nil 1 2) -> (2 1)
- conj a map value into a map/phm merges its entries ((conj {:a 0} {:b 1}) ->
  {:a 0 :b 1}); a [k v] vector/pair still adds one entry.

conj.cljc 14/6/5 -> 21/3/1. clojure-test-suite pass 3681->3691, errors 105->98.
spec: seq/conj-edge-cases (8). jpm test green.

test/integration/clojure-test-suite-test.janet

@@ -18,7 +18,7 @@
 
 # Baseline: assertions Jolt currently passes across the suite. Raise as Jolt
 # improves so a regression (previously-passing assertion breaking) is caught.
-(def baseline-pass 3680)
+(def baseline-pass 3690)
 # A file is "clean" when it ran with zero failures AND zero errors.
 (def baseline-clean-files 45)
 # Per-file wall-clock budget (seconds). Normal files finish in well under 1s;