I removed old mezuzah , but inside was empty. What should I do with the old case ?

ב"ה Shalom The general ruling is if the Mezuza case was used to honor the Mezuza, then we see the Mezuza case as "Tashmish Kedusha"=תשמיש קדושה , meaning that it served something holy and it requires placing it in "Geniza" where people place worn siddurim , Chumashim and the like. However, if the case is a simple casing which was merely to protect the Mezuza , then it does not require Geniza because it hasn't earned the status of a "Tashmish Kedusha". See שולחן ערוך או"ח סי' קנד סע' ג, המשנה ברורה (ס"ק יד) : All the best