Dictionary merge: Correctly drop empty data tags

This allows for TagLib to handle artwork reading where the file built-in
readers fail, such as the FFmpeg reader, which would require parsing the
stream data for artwork packets, a really wacky convention to have.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Utils/NSDictionary+Merge.m

@@ -20,6 +20,9 @@
 			} else if([objTarget isKindOfClass:[NSNumber class]]) {
 				NSNumber *val = (NSNumber *)objTarget;
 				isEmpty = [val isEqualTo:@(0)];
+			} else if([objTarget isKindOfClass:[NSData class]]) {
+				NSData *val = (NSData *)objTarget;
+				isEmpty = [val length] == 0;
 			}
 			if(isEmpty) {
 				[result setObject:obj forKey:key];