patch 8.2.2291: Vim9: cannot use "null" for v:null

Problem:    Vim9: cannot use "null" for v:null.
Solution:   Support "null" like "true" and "false". (closes #7495)
diff --git a/src/evalvars.c b/src/evalvars.c
index c6b03e9..42ff828 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -2072,8 +2072,8 @@
     {
 	case VVAL_FALSE: return in_vim9script() ? "false" : "v:false";
 	case VVAL_TRUE:  return in_vim9script() ? "true" : "v:true";
+	case VVAL_NULL:  return in_vim9script() ? "null" : "v:null";
 	case VVAL_NONE:  return "v:none";
-	case VVAL_NULL:  return "v:null";
     }
     internal_error("get_var_special_name()");
     return "42";