SNCF is a little bit special
in announcing their timetables in advance. As you can see in the leaflet, the common European timetable change (which would be on the second Sunday of June) is ignored, instead the timings are valid until 7th July.
Many trains are included in the official timetables at rather short notice, this is mainly due to track renewal which is carried out throughout the country. Of course this makes planning ahead not very easy.
Also, timings of many many trains change throughout the year on several occasions, often by only a few minutes - again this is due to track works.
Sadly information on whether a specific train will run on a specific date on a specific time is hard to get - at least SNCF now offers these leaflets for many Intercity services, which was not always the case.
Regarding your Nice - Paris night train I am quite sure that it will run daily during the summer, although like I tried to point out above there is no guarantee on that. You can only wait some time and check the schedules online from time to time whether there are any changes or not.