With SetNullBlock you can set the default value to put if the block doesn't get parsed.
the default is to wipe it out, but you can set it up to put a custom string.

what happens if we don't parse the subblocks?
sub blocks begins here ==> [ ] <== sub blocks ends here.

what happens if we parse them? :)
sub blocks begins here ==> [ this is subblock1 _PARSED_ this is subblock2 _PARSED_ ] <== sub blocks ends here.

ok.. set_null_block("block not parsed!") coming
sub blocks begins here ==> [ block not parsed! block not parsed! ] <== sub blocks ends here.

ok.. custom nullblocks.. set_null_block('subblock1 not parsed!', 'main.block.subblock1')
sub blocks begins here ==> [ subblock1 not parsed! block not parsed! ] <== sub blocks ends here.


..got it? :)