Here is an idea, put another column for status, one of the status will be working status and every other status will be as described. Only working status will have data in the "start time" and "end time" columns. In my opinion this is the best solution and allows for better search capabilities, cleaner database and more comprehensive readability.
However, if you absolutely want to, and/or have any reason on why you can't have an additional column, you can always store your time as text.
PS: Another tip for your database is to drop the date column and store both times in DATETIME format, it may range from unlikely to nearly impossible depending on what job shifts you are storing in the database, but it is possible to start a shift on one day and end it the next day, and even if you think you won't ever need it, it is good practice and makes the database more resilient. If you had to change it in the future it would be a pain to do so.