patch 8.2.1802: Vim9: crash with unterminated dict

Problem:    Vim9: crash with unterminated dict. (Dhiraj Mishra)
Solution:   Return empty string instead of NULL. (closes #7084)

src/testdir/test_vim9_expr.vim

@@ -1819,6 +1819,8 @@ def Test_expr7_dict()
   CheckDefExecFailure(['var x: dict<number> = #{a: "x", b: 134}'], 'E1012:', 1)
   CheckDefExecFailure(['var x: dict<string> = #{a: 234, b: "1"}'], 'E1012:', 1)
   CheckDefExecFailure(['var x: dict<string> = #{a: "x", b: 134}'], 'E1012:', 1)
+
+  CheckDefFailure(['var x = ({'], 'E723:', 2)
 enddef
 
 def Test_expr7_dict_vim9script()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1802,
 /**/
     1801,
 /**/

src/vim9compile.c

@@ -2822,7 +2822,10 @@ compile_dict(char_u **arg, cctx_T *cctx, int literal)
 
 failret:
     if (*arg == NULL)
+    {
 	semsg(_(e_missing_dict_end), _("[end of lines]"));
+	*arg = (char_u *)"";
+    }
     dict_unref(d);
     return FAIL;
 }