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".
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 9f6cc5d..d908e64 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -3313,9 +3313,8 @@
 			break;
 		    default:
 			tv->v_type = VAR_STRING;
-			tv->vval.v_string = vim_strsave(
-				iptr->isn_arg.string == NULL
-					? (char_u *)"" : iptr->isn_arg.string);
+			tv->vval.v_string = iptr->isn_arg.string == NULL
+				    ? NULL : vim_strsave(iptr->isn_arg.string);
 		}
 		break;