Fix "string is undefined" export error

Regression from a549a64de9

chrome/content/zotero/xpcom/date.js

@@ -263,7 +263,9 @@ Zotero.Date = new function(){
 			order: ''
 		};
 		
-		string = Zotero.Utilities.trimInternal(string.toString());
+		if (typeof string == 'string' || typeof string == 'number') {
+			string = Zotero.Utilities.trimInternal(string.toString());
+		}
 		
 		// skip empty things
 		if(!string) {

test/tests/dateTest.js

@@ -157,6 +157,31 @@ describe("Zotero.Date", function() {
 	})
 	
 	describe("#strToDate()", function () {
+		it("should return object without date parts for null", function () {
+			var o = Zotero.Date.strToDate(null);
+			assert.notProperty(o, 'year');
+		});
+		
+		it("should return object without date parts for undefined", function () {
+			var o = Zotero.Date.strToDate();
+			assert.notProperty(o, 'year');
+		});
+		
+		it("should return object without date parts for false", function () {
+			var o = Zotero.Date.strToDate(false);
+			assert.notProperty(o, 'year');
+		});
+		
+		it("should return object without date parts for empty string", function () {
+			var o = Zotero.Date.strToDate('');
+			assert.notProperty(o, 'year');
+		});
+		
+		it("should return object without date parts for blank string", function () {
+			var o = Zotero.Date.strToDate(' ');
+			assert.notProperty(o, 'year');
+		});
+		
 		it("should work in translator sandbox", function* () {
 			var item = createUnsavedDataObject('item');
 			item.libraryID = Zotero.Libraries.userLibraryID;