fix(json-enc): restore hx-vals keys dropped by empty-array FormData encoding

src/json-enc/json-enc.js

@@ -27,7 +27,11 @@
       })
 
       const vals = api.getExpressionVars(elt)
-      Object.keys(object).forEach(function(key) {
+      // A hx-vals/hx-vars key whose value serializes to nothing in FormData (e.g. an
+      // empty array) has no entry in `object` yet, so the key set to restore over must
+      // be the union of both, not just what FormData produced.
+      const keys = new Set(Object.keys(object).concat(Object.keys(vals)))
+      keys.forEach(function(key) {
         // FormData encodes values as strings, restore hx-vals/hx-vars with their initial types
         object[key] = Object.hasOwn(vals, key) ? vals[key] : object[key]
       })

src/json-enc/test/ext/json-enc.js

@@ -182,6 +182,25 @@ describe('json-enc extension', function() {
     form.innerHTML.should.equal('OK')
   })
 
+  it('keeps hx-vals keys whose value serializes to nothing in FormData', function() {
+    var values = {}
+    this.server.respondWith('POST', '/test', function(xhr) {
+      values = JSON.parse(xhr.requestBody)
+      xhr.respond(200, {}, 'clicked')
+    })
+
+    make(`<form hx-ext="json-enc" hx-post="/test" hx-vals="js:{'firstLevel': [], 'secondLevel': {'values': []}}">
+             <button id="btn" type="submit">Submit</button>
+          </form>`)
+    var btn = byId('btn')
+    btn.click()
+    this.server.respond()
+
+    values.should.have.keys('firstLevel', 'secondLevel')
+    chai.assert.deepEqual(values.firstLevel, [])
+    chai.assert.deepEqual(values.secondLevel, { values: [] })
+  })
+
   it('handles multiple select', function() {
     this.server.respondWith('POST', '/test', function(xhr) {
       var values = JSON.parse(xhr.requestBody)