refactor execSelector

This commit fully separates the normalization of `class` and `className` from the merging of `vnode.attrs` and `vnode.state.attrs`, which were previously handled together in the same code path.
This simplifies conditional branches and improves bundle size and performance.

Author: kfule

render/hyperscript.js

@@ -51,24 +51,21 @@ function execSelector(state, vnode) {
 		return vnode
 	}
 
-	var hasClass = hasOwn.call(attrs, "class")
-	var className = hasClass ? attrs.class : attrs.className
+	if (hasOwn.call(attrs, "class")) {
+		if (attrs.class != null) attrs.className = attrs.class
+		attrs.class = null
+	}
 
 	if (state.attrs !== emptyAttrs) {
+		var className = attrs.className
 		attrs = Object.assign({}, state.attrs, attrs)
 
-		if (className != null || state.attrs.className != null) attrs.className =
+		if (state.attrs.className != null) attrs.className =
 			className != null
-				? state.attrs.className != null
-					? String(state.attrs.className) + " " + String(className)
-					: className
+				? String(state.attrs.className) + " " + String(className)
 				: state.attrs.className
-	} else {
-		if (className != null) attrs.className = className
 	}
 
-	if (hasClass) attrs.class = null
-
 	// workaround for #2622 (reorder keys in attrs to set "type" first)
 	// The DOM does things to inputs based on the "type", so it needs set first.
 	// See: https://github.com/MithrilJS/mithril.js/issues/2622