JSON Time wrong formatted

in the database are several time-columns with values like 16:15:00 and 00:00:00.

We are getting the data with the default JSON-Rest integration of cakephp3.

The output is:
" sunday_end1": null,
"saturday_end1": null,
"monday_start2": "2017-10-11T23:45:00+0000",
"monday_end2": "2017-10-11T23:30:00+0000",

Here I have two questions:
The value in the database is:
sunday_end1: 00:00:00
staurday_end1: 00:00:00
monday_start2: 23:45:00
monday_end2: 23:45:00

How can I disable the date-output? The time is the only relevant thing.
How can I enable the output “00:00:00” instead of null?

Of course I could parse the null-Field with 00:00:00 when using it, but a correct output would be nice.

I’m using Cakephp3, the Database-Fields have the type “time”.

Thank you very much