remove expansions from objects when we know the value being expanded …

…is nullish. Closes #1004

lib/compress/index.js

@@ -7469,7 +7469,7 @@ def_optimize(AST_Array, function(self, compressor) {
     return self;
 });
 
-function inline_object_prop_spread(props) {
+function inline_object_prop_spread(props, compressor) {
     for (var i = 0; i < props.length; i++) {
         var prop = props[i];
         if (prop instanceof AST_Expansion) {
@@ -7487,6 +7487,9 @@ function inline_object_prop_spread(props) {
                 // non-iterable value silently does nothing; it is thus safe
                 // to remove. AST_String is the only iterable AST_Constant.
                 props.splice(i, 1);
+            } else if (is_nullish(expr, compressor)) {
+                // Likewise, null and undefined can be silently removed.
+                props.splice(i, 1);
             }
         }
     }
@@ -7497,7 +7500,7 @@ def_optimize(AST_Object, function(self, compressor) {
     if (optimized !== self) {
         return optimized;
     }
-    inline_object_prop_spread(self.properties);
+    inline_object_prop_spread(self.properties, compressor);
     return self;
 });