fix #15934 and #15620 (#15938) [backport:1.4]

* make workaround for #15934 and #15620
* add testcase for #9754

lib/core/typeinfo.nim

@@ -327,7 +327,10 @@ proc setPointer*(x: Any, y: pointer) =
   ## ``akString``, ``akCString``, ``akProc``, ``akRef``, ``akPtr``,
   ## ``akPointer``, ``akSequence``.
   assert x.rawType.kind in pointerLike
+  if y != nil:
     genericAssign(x.value, y, x.rawType)
+  else:
+    cast[ppointer](x.value)[] = nil
 
 proc fieldsAux(p: pointer, n: ptr TNimNode,
                ret: var seq[tuple[name: cstring, any: Any]]) =

tests/stdlib/t9754.nim

@@ -0,0 +1,6 @@
+discard """
+  joinable: false
+"""
+
+import tmarshal
+import tparsesql

tests/stdlib/tmarshal.nim

@@ -4,6 +4,9 @@ true
 true
 alpha 100
 omega 200
+Some(null)
+None[JsonNode]
+(numeric: "")
 '''
 joinable: false
 """
@@ -123,3 +126,25 @@ var foo = Foo(a2: "", a4: @[], a6: @[1])
 foo.a6.setLen 0
 doAssert $$foo == """{"a1": "", "a2": "", "a3": [], "a4": [], "a5": [], "a6": []}"""
 testit(foo)
+
+import options, json
+
+# bug #15934
+block:
+  let
+    a1 = some(newJNull())
+    a2 = none(JsonNode)
+  echo ($$a1).to[:Option[JsonNode]]
+  echo ($$a2).to[:Option[JsonNode]]
+
+
+#  bug #15620
+block:
+  let str = """{"numeric": null}"""
+
+  type
+    LegacyEntry = object
+      numeric: string
+
+  let test = to[LegacyEntry](str)
+  echo test