Use NULL instead of nil for error pointer parameter

Keeping with the style.

Author: indragiek

FastImageCache/FICImageTable.m

@@ -687,7 +687,7 @@ - (void)saveMetadata {
 
 - (void)_loadMetadata {
     NSString *metadataFilePath = [[_filePath stringByDeletingPathExtension] stringByAppendingPathExtension:FICImageTableMetadataFileExtension];
-    NSData *metadataData = [NSData dataWithContentsOfURL:[NSURL fileURLWithPath:metadataFilePath] options:NSDataReadingMappedAlways error:nil];
+    NSData *metadataData = [NSData dataWithContentsOfURL:[NSURL fileURLWithPath:metadataFilePath] options:NSDataReadingMappedAlways error:NULL];
     if (metadataData != nil) {
         NSDictionary *metadataDictionary = (NSDictionary *)[NSPropertyListSerialization propertyListWithData:metadataData options:0 format:NULL error:NULL];
         NSDictionary *formatDictionary = [metadataDictionary objectForKey:FICImageTableFormatKey];