If the user has 12h time and sets a properly formatted but invalid time,
Calendar would crash because `date_time` would be `NULL`. This would
specifically happen if the time was set over 23:59 AM or 11:59 PM (ex:
19:00 PM).
By breaking if `date_time` is `NULL`, the crash can be avoided.
Closes#1602
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/848>
When clicking the previous/next buttons, the view might not be aligned
to the start of the month, which can cause confusion.
Clamping the date to the first of the month removes this confusion by
being consistent. This also matches the behavior of the previous/next
buttons in the main toolbar.
Closes#1606
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/821>