fix(react): treat valueless boolean attrs as true in codegen

When compiling templates for React/RN, attributes written without a
value (e.g. <scroll-view scroll-y>) had their value parsed as
undefined, which JSON.stringify silently drops. Now such attributes
correctly emit `true` in the generated render function, matching
miniprogram boolean-attribute semantics.

Attributes with an explicit value (e.g. scroll-y="{{undefined}}") are
unchanged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mackwang112

packages/webpack-plugin/lib/template-compiler/gen-node-react.js

@@ -90,7 +90,7 @@ function genNode (node, isRoot = false) {
             if (node.attrsList.length) {
               const attrs = []
               node.attrsList && node.attrsList.forEach(({ name, value }) => {
-                const attrExp = attrExpMap[name] ? attrExpMap[name] : s(value)
+                const attrExp = attrExpMap[name] ? attrExpMap[name] : (value === undefined ? 'true' : s(value))
                 attrs.push(`${mapAttrName(name)}: ${attrExp}`)
               })
               exp += `, { ${attrs.join(', ')} }`