ipc: send 'percent': null when percent is not relevant for the container

src/ipc.c

@@ -193,7 +193,9 @@ void dump_node(yajl_gen gen, struct Con *con, bool inplace_restart) {
     }
 
     ystr("percent");
-    y(double, con->percent);
+    if (con->percent == 0.0)
+        y(null);
+    else y(double, con->percent);
 
     ystr("urgent");
     y(integer, con->urgent);

testcases/t/16-nestedcons.t

@@ -25,7 +25,7 @@ my $expected = {
     window_rect => ignore(),
     geometry => ignore(),
     swallows => ignore(),
-    percent => 0,
+    percent => undef,
     layout => 'default',
     focus => ignore(),
     focused => 0,