UniquePersonList: check for null arguments more thoroughly

Some methods do not explicitly check that their arguments are non-null
with `requireNonNull(...)`.

Let's fix them to be consistent with the rest of the code base.

src/main/java/seedu/address/model/person/UniquePersonList.java

@@ -52,7 +52,7 @@ public void add(Person toAdd) {
      * The person identity of {@code editedPerson} must not be the same as another existing person in the list.
      */
     public void setPerson(Person target, Person editedPerson) {
-        requireNonNull(editedPerson);
+        requireAllNonNull(target, editedPerson);
 
         int index = internalList.indexOf(target);
         if (index == -1) {
@@ -78,6 +78,7 @@ public void remove(Person toRemove) {
     }
 
     public void setPersons(UniquePersonList replacement) {
+        requireNonNull(replacement);
         this.internalList.setAll(replacement.internalList);
     }