Sort empty rows last for Date/Year columns when descending

Date columns start in descending order, so put empty rows at end

Also remove a seemingly unnecessary exception for Title when not the
primary sort and just always sort it with empty rows first

chrome/content/zotero/itemTree.jsx

@@ -1242,20 +1242,14 @@ var ItemTree = class ItemTree extends LibraryTree {
 				? `for ${itemIDs.length} ` + Zotero.Utilities.pluralize(itemIDs.length, ['item', 'items'])
 				: ""));
 		
-		// Set whether rows with empty values should be displayed last,
+		// Set whether rows with empty values should sort at the beginning
-		// which may be different for primary and secondary sorting.
+		var emptyFirst = {
-		var emptyFirst = {};
+			title: true,
-		switch (primaryField) {
+			
-		case 'title':
+			// Date columns start descending, so put empty rows at end
-			emptyFirst.title = true;
+			date: true,
-			break;
+			year: true,
-		
+		};
-		// When sorting by title we want empty titles at the top, but if not
-		// sorting by title, empty titles should sort to the bottom so that new
-		// empty items don't get sorted to the middle of the items list.
-		default:
-			emptyFirst.title = false;
-		}
 		
 		// Cache primary values while sorting, since base-field-mapped getField()
 		// calls are relatively expensive