vim-patch:8.2.0619: null dict is not handled like an empty dict

Problem:    Null dict is not handled like an empty dict.
Solution:   Fix the code and add tests. (Yegappan Lakshmanan, closes vim/vim#5968)

ea04a6e8ba

Nvim doesn't support modifying NULL list, so comment out a line.

src/nvim/eval.c

@@ -1449,6 +1449,7 @@ char *get_lval(char *const name, typval_T *const rettv, lval_T *const lp, const
           key[len] = prevval;
         }
         if (wrong) {
+          tv_clear(&var1);
           return NULL;
         }
       }