Fix typeSym.getImpl for ref types (#13752)

* Fix typeSym.getImpl for ref types

* Fix a codegen issue affecting the test suite of nim-beacon-chain

* Fix tests/stdlib/tjsonmacro

To understand the fix better it may help to take a look
at the history of the replaced code.

The nil check that is removed in this commit was introduced
in another fix that failed to identify the root cause of the
issue - namely that we allow an object type to exist for which
no ast is present:

https://github.com/nim-lang/Nim/pull/9601/files

The original intention of the code is more obvious here:

https://github.com/nim-lang/Nim/pull/9538/files

compiler/vmdeps.nim

@@ -159,10 +159,10 @@ proc mapTypeToAstX(cache: IdentCache; t: PType; info: TLineInfo;
   of tyObject:
     if inst:
       result = newNodeX(nkObjectTy)
-      if t.sym.ast != nil:
-        result.add t.sym.ast[2][0].copyTree  # copy object pragmas
-      else:
-        result.add newNodeI(nkEmpty, info)
+      var objectDef = t.sym.ast[2]
+      if objectDef.kind == nkRefTy:
+        objectDef = objectDef[0]
+      result.add objectDef[0].copyTree  # copy object pragmas
       if t[0] == nil:
         result.add newNodeI(nkEmpty, info)
       else:  # handle parent object