[FIX] web: TempusDominus unsafe access to config

During TempusDominus autobinding to fields matching its default classes
(like `.datetimepicker-input` used by our DatePicker widget) and no
config was previously provided to the library (like when we disable it
on mobile), an unsafe access to the config's `_options` property results
into an error as the config is `undefined`.

This commit fixes it by first checking for config existence before
attempting to access its property.

Note: as this is a fix inside a library, a comment is added to make
clear. Also similar fixes where already done in the same file.

opw-2242880