fixes #21638; fromJson should support empty objects (#21641)

* fixes #21638; `fromJson` should supports empty objects

* complete the logic

(cherry picked from commit 5e016e4466)

lib/std/jsonutils.nim

@@ -89,19 +89,24 @@ macro getDiscriminants(a: typedesc): seq[string] =
   let sym = a[1]
   let t = sym.getTypeImpl
   let t2 = t[2]
-  doAssert t2.kind == nnkRecList
-  result = newTree(nnkBracket)
-  for ti in t2:
-    if ti.kind == nnkRecCase:
-      let key = ti[0][0]
-      let typ = ti[0][1]
-      result.add newLit key.strVal
-  if result.len > 0:
+  case t2.kind
+  of nnkEmpty: # allow empty objects
     result = quote do:
-      @`result`
+        seq[string].default
+  of nnkRecList:
+    result = newTree(nnkBracket)
+    for ti in t2:
+      if ti.kind == nnkRecCase:
+        let key = ti[0][0]
+        result.add newLit key.strVal
+    if result.len > 0:
+      result = quote do:
+        @`result`
+    else:
+      result = quote do:
+        seq[string].default
   else:
-    result = quote do:
-      seq[string].default
+    doAssert false, "unexpected kind: " & $t2.kind
 
 macro initCaseObject(T: typedesc, fun: untyped): untyped =
   ## does the minimum to construct a valid case object, only initializing