patch 8.2.4534: Vim9: "is" operator with empty string and null returns true

Problem:    Vim9: "is" operator with empty string and null returns true.
Solution:   Consider empty string and null to be different for "is".

src/testdir/test_vim9_expr.vim

@@ -801,6 +801,13 @@ def Test_expr4_compare_null()
       assert_false(null_string != null)
       assert_false(v:null != test_null_string())
       assert_false(null != null_string)
+
+      assert_true(null_string is test_null_string())
+      assert_false(null_string is '')
+      assert_false('' is null_string)
+      assert_false(null_string isnot test_null_string())
+      assert_true(null_string isnot '')
+      assert_true('' isnot null_string)
   END
   v9.CheckDefAndScriptSuccess(lines)
   unlet g:null_dict

src/typval.c

@@ -1583,9 +1583,23 @@ typval_compare_string(
 	i = ic ? MB_STRICMP(s1, s2) : STRCMP(s1, s2);
     switch (type)
     {
-	case EXPR_IS:
+	case EXPR_IS:	    if (in_vim9script())
+			    {
+				// Really check it is the same string, not just
+				// the same value.
+				val = tv1->vval.v_string == tv2->vval.v_string;
+				break;
+			    }
+			    // FALLTHROUGH
 	case EXPR_EQUAL:    val = (i == 0); break;
-	case EXPR_ISNOT:
+	case EXPR_ISNOT:    if (in_vim9script())
+			    {
+				// Really check it is not the same string, not
+				// just a different value.
+				val = tv1->vval.v_string != tv2->vval.v_string;
+				break;
+			    }
+			    // FALLTHROUGH
 	case EXPR_NEQUAL:   val = (i != 0); break;
 	case EXPR_GREATER:  val = (i > 0); break;
 	case EXPR_GEQUAL:   val = (i >= 0); break;

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4534,
 /**/
     4533,
 /**/

src/vim9execute.c

@@ -3313,9 +3313,8 @@ exec_instructions(ectx_T *ectx)
 			break;
 		    default:
 			tv->v_type = VAR_STRING;
-			tv->vval.v_string = vim_strsave(
+			tv->vval.v_string = iptr->isn_arg.string == NULL
-				iptr->isn_arg.string == NULL
+				    ? NULL : vim_strsave(iptr->isn_arg.string);
-					? (char_u *)"" : iptr->isn_arg.string);
 		}
 		break;