Remap source maps in JS DCE. Improve JS DCE error logging

See KT-19821

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsCallChecker.kt

@@ -92,7 +92,7 @@ class JsCallChecker(
             val parserScope = JsFunctionScope(JsRootScope(JsProgram()), "<js fun>")
             val statements = parse(code, errorReporter, parserScope, reportOn.containingFile?.name ?: "<unknown file>")
 
-            if (statements.isEmpty()) {
+            if (statements == null || statements.isEmpty()) {
                 context.trace.report(ErrorsJs.JSCODE_NO_JAVASCRIPT_PRODUCED.on(argument))
             }
         } catch (e: AbortParsingException) {