We don't set null_value to 0 in the Item_func_compress::val_str() for 
not-NULL results.


mysql-test/r/func_compress.result:
  Fix for bug #23254: COMPRESS(NULL) makes all futher COMPRESS() calls on same Item return NULL
    - test result.
mysql-test/t/func_compress.test:
  Fix for bug #23254: COMPRESS(NULL) makes all futher COMPRESS() calls on same Item return NULL
    - test case.
sql/item_strfunc.cc:
  Fix for bug #23254: COMPRESS(NULL) makes all futher COMPRESS() calls on same Item return NULL
    - set null_value.