Dependencies on binary items are not triggered

[denisdefreyne]

When the content of a binary item changes, items that depend on this binary item are not recompiled.

Steps to reproduce

1. Create a binary item A
2. Create an item B that depends on this binary item
3. Compile
4. Change contents of item A
5. Compile

Expected behavior

Item B is recompiled (identical or updated is shown)

Actual behavior

Item B is not recompiled.

Details

The filesystem data source’s #content_checksum_data_for defaults to '' for the content. It should return nil instead, so that the checksum is calculated properly.

CC @moll