Add isValid or equivalent to NotesCalendarEntry

When you use NotesCalendar#getEntry, it will always return a NotesCalendarEntry object even if the UID is not found. An invalid UID will result in an object that throws an exception when you try to do basically anything with it. While documented, this is... odd behavior, especially since the exceptions all come from internal implementation functions and are like "Entry not found in index" or "Invalid or nonexistent document", not something describing the immediate situation.

So, in light of that, it'd at least be useful to have an isValid() or equivalent method to do this check without the thrown exception, since having to do a try/catch on every use of getEntry is cumbersome.