patch 8.2.4530: making comparison with null work changes legacy behavior

Problem:    Making comparison with null work changes legacy behavior.
Solution:   Only use the better comparison in Vim9 script. (closes #9910)

src/testdir/test_expr.vim

@@ -181,6 +181,38 @@ func Test_loop_over_null_list()
   call v9.CheckScriptFailure(lines, 'E121: Undefined variable: null_list')
 endfunc
 
+func Test_compare_with_null()
+  let s:value = v:null
+  call assert_true(s:value == v:null)
+  let s:value = v:true
+  call assert_false(s:value == v:null)
+  let s:value = v:none
+  call assert_false(s:value == v:null)
+  let s:value = 0
+  call assert_true(s:value == v:null)
+  if has('float')
+    let s:value = 0.0
+    call assert_true(s:value == v:null)
+  endif
+  let s:value = ''
+  call assert_false(s:value == v:null)
+  let s:value = 0z
+  call assert_false(s:value == v:null)
+  let s:value = []
+  call assert_false(s:value == v:null)
+  let s:value = {}
+  call assert_false(s:value == v:null)
+  let s:value = function('len')
+  call assert_false(s:value == v:null)
+  if has('job')
+    let s:value = test_null_job()
+    call assert_true(s:value == v:null)
+    let s:value = test_null_channel()
+    call assert_true(s:value == v:null)
+  endif
+  unlet s:value
+endfunc
+
 func Test_setreg_null_list()
   let lines =<< trim END
       call setreg('x', test_null_list())

src/typval.c

@@ -1381,7 +1381,7 @@ typval_compare_list(
 }
 
 /*
- * Compare v:null/v:none with another type.  Return TRUE if the value is NULL.
+ * Compare v:null with another type.  Return TRUE if the value is NULL.
  */
     int
 typval_compare_null(typval_T *tv1, typval_T *tv2)
@@ -1417,6 +1417,9 @@ typval_compare_null(typval_T *tv1, typval_T *tv2)
 	    default: break;
 	}
     }
+    if (!in_vim9script())
+	return FALSE;  // backwards compatible
+
     semsg(_(e_cannot_compare_str_with_str),
 			 vartype_name(tv1->v_type), vartype_name(tv2->v_type));
     return MAYBE;

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4530,
 /**/
     4529,
 /**/