Fixed: CPDateFormatter stringFromDate:"" did not return a default date.

In Cocoa formatting an empty string actually results in a particular date. For consistency we should handle an empty string the same way.

Refs #2030.

Foundation/CPDateFormatter.j

@@ -1104,9 +1104,6 @@ var defaultDateFormatterBehavior = CPDateFormatterBehavior10_4,
 */
 - (CPDate)dateFromString:(CPString)aString
 {
-    if (aString == nil)
-        return nil;
-
     var format;
 
     if (_dateFormat != nil)
@@ -1224,7 +1221,11 @@ var defaultDateFormatterBehavior = CPDateFormatterBehavior10_4,
 */
 - (CPDate)_dateFromString:(CPString)aString format:(CPString)aFormat
 {
-    if (aString == nil || aFormat == nil)
+    // Interpret @"" as the date 2000-01-01 00:00:00 +0000, like in Cocoa. No idea why they picked this particular date.
+    if (!aString)
+        return [[CPDate alloc] initWithTimeIntervalSinceReferenceDate:-31622400];
+    
+    if (aFormat == nil)
         return nil;
 
     var currentToken = [CPString new],

Tests/Foundation/CPDateFormatterTest.j

@@ -450,7 +450,7 @@
     [_dateFormatter setDateFormat:@""];
     var result = [_dateFormatter dateFromString:@""];
 
-    [self assert:[result isEqualToDate:[[CPDate alloc] initWithString:@"2000-01-01 08:00:00 +0000"]] equals:YES];
+    [self assert:[result isEqualToDate:[[CPDate alloc] initWithString:@"2000-01-01 00:00:00 +0000"]] equals:YES];
 }
 
 - (void)testDateFromStringTokeny
@@ -1180,6 +1180,19 @@
     [self assertTrue:date === nil];
 }
 
+- (void)testGetObjectValueForEmptyStringReturnsReferenceDate
+{
+    [_dateFormatter setDateFormat:@"d mm"];
+
+    var date = nil,
+        error = @"",
+        result = [_dateFormatter getObjectValue:@ref(date) forString:@"" errorDescription:@ref(error)];
+
+    [self assertTrue:result];
+    [self assert:[[CPDate alloc] initWithString:@"2000-01-01 00:00:00 +0000"] equals:date];
+    [self assert:error equals:@""];
+}
+
 - (void)testGetObjectValueReturnYes
 {
     [_dateFormatter setDateFormat:@"d mm"];