CLJS-3255: cljs.build.api/build doesn't work with single arity / 2-arity with nil

While actually supported make a small change so that `source` argument to cljs.build/build
can actually be nil in all cases. If `:main` is supplied, `:optimizations` `:none`, and
source nil, go ahead supply source as that will always be what the user intended.

Note cljs.cli already fixed this problem some time ago but at a higher level.

Author: swannodette

src/main/clojure/cljs/closure.clj

@@ -3109,11 +3109,15 @@
              (util/debug-prn "Options passed to ClojureScript compiler:" (pr-str opts)))
            (let [one-file? (and (:main opts)
                                 (#{:advanced :simple :whitespace} (:optimizations opts)))
-                 source (if one-file?
+                 source (if (or one-file?
+                                ;; if source is nil, :main is supplied, :optimizations :none,
+                                ;; fix up source for the user, see CLJS-3255
+                                (and (nil? source) (:main opts) (= :none (:optimizations opts))))
                           (let [main (:main opts)
                                 uri  (:uri (cljs-source-for-namespace main))]
                             (assert uri (str "No file for namespace " main " exists"))
                             uri)
+                          ;; old compile directory behavior, or code-splitting
                           source)
                  compile-opts (if one-file?
                                 (assoc opts :output-file (:output-to opts))

src/test/clojure/cljs/build_api_tests.clj

@@ -719,3 +719,15 @@
     (test/delete-out-files out)
     (build/build (build/inputs (io/file inputs "trivial/core.cljs")) opts cenv)
     (is (< (.length out-file) 10000))))
+
+(deftest cljs-3255-nil-inputs-build
+  (let [out (.getPath (io/file (test/tmp-dir) "3255-test-out"))
+        out-file (io/file out "main.js")
+        opts {:main 'trivial.core
+              :output-to (.getPath out-file)
+              :output-dir out
+              :language-in :es6
+              :optimizations :none}
+        cenv (env/default-compiler-env)]
+    (test/delete-out-files out)
+    (build/build nil opts cenv)))