css empty rule should be eliminate

[susiwen8]

In css file there are some empty rulesets, esbuild fail to remove them in bundle and minify.

.a {
    
}
.b { color: red;}

@keyframes test {
    from {}
    to {
        color: red;
    }
}

it should be

/* in webpack */
.b{color:red}@keyframes test{to{color:red}}

but esbuild save them

/* in esbuild */
.a{}.b{color:red}@keyframes test{0%{}to{color:red}}