patch 9.0.1796: Vim9 problems with null_objects

Problem:  Vim9 problems with null_objects
Solution: Vim9 improve null_object usage

Fix "xvar == null", where xvar might have been assigned null_object.

Fix compilation failure: "var o2: C = null_object".

closes: #12890

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>
diff --git a/src/vim9instr.c b/src/vim9instr.c
index 941adf8..3e9c6b6 100644
--- a/src/vim9instr.c
+++ b/src/vim9instr.c
@@ -662,11 +662,11 @@
 /*
  * Generate an ISN_PUSHOBJ instruction.  Object is always NULL.
  */
-    static int
+    int
 generate_PUSHOBJ(cctx_T *cctx)
 {
     RETURN_OK_IF_SKIP(cctx);
-    if (generate_instr_type(cctx, ISN_PUSHOBJ, &t_any) == NULL)
+    if (generate_instr_type(cctx, ISN_PUSHOBJ, &t_object) == NULL)
 	return FAIL;
     return OK;
 }