Use Objects.equals instead of writing same logic

In few places in the code there is duplicated, not needed logic that does an object equality check treating objects as equal if both reference a null.

For example, Condition class' equals method does it few times:

 if (onsetDate != null ? !onsetDate.equals(condition.onsetDate) : condition.onsetDate != null)
{ return false; }

Code like this can be replaced with the Objects#equals(Object a, Object b)

It's a small thing, but removing custom code and using something already available is a good practice. These checks are also error prone since detailed comparison is being made together with lots of negations.