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)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 373d76c..a6bdcc3 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -2822,7 +2822,10 @@
 
 failret:
     if (*arg == NULL)
+    {
 	semsg(_(e_missing_dict_end), _("[end of lines]"));
+	*arg = (char_u *)"";
+    }
     dict_unref(d);
     return FAIL;
 }