fix corner case in AST_For.init (#1652)

Enforce `null` as value for empty initialisation blocks.

fixes #1648

lib/compress.js

@@ -2327,6 +2327,7 @@ merge(Compressor.prototype, {
     };
 
     OPT(AST_For, function(self, compressor){
+        if (is_empty(self.init)) self.init = null;
         if (!compressor.option("loops")) return self;
         if (self.condition) {
             var cond = self.condition.evaluate(compressor);

lib/output.js

@@ -799,7 +799,7 @@ function OutputStream(options) {
         output.print("for");
         output.space();
         output.with_parens(function(){
-            if (self.init && !(self.init instanceof AST_EmptyStatement)) {
+            if (self.init) {
                 if (self.init instanceof AST_Definitions) {
                     self.init.print(output);
                 } else {