StatisticsDialog: exclude empty histories

If a YearHistory contains zero recorded dates, then StatisticsDialog
throws a NoSuchElementException from a call to java.util.SortedSet#first.

Exclude all histories, which have zero days from the statistics
gathering to fix this exception.

CHANGELOG

@@ -3,6 +3,8 @@ Features
 Bugfixes
 - Hiding last visible habit from "Reorder habits" dialog caused an exception to
   be thrown, which has been corrected.
+- Gathering of statistics for habits with zero recorded dates used to throw
+  an exception, which has been corrected.
 Improvements
 - Version of Resoday shown in the "About" dialog no longer starts with a "v".
 

src/main/java/dev/andrybak/resoday/gui/stats/StatisticsDialog.java

@@ -50,6 +50,9 @@ private static JTable createStatsTable(Histories histories) {
 		);
 		histories.forEachHistory(h -> {
 			NavigableSet<LocalDate> ns = h.toNavigableSet();
+			if (ns.isEmpty()) {
+				return;
+			}
 			tableModel.addRow(new Object[]{
 				h.getName(),
 				ns.size(),