Since EZP-25276 BC might not be respected.

Comment from Gaetano who found the issue:
This (I think) causes a regression in the testsuite of ezmigrationbundle: https://travis-ci.org/kaliop-uk/ezmigrationbundle/builds/223343441 (notice that tests pass on older ez, but not on ezplatform. tests where passing yesterday on all platforms...).

The error message I get is:

Failed to parse time string (@) at position 0 (@): Unexpected character in file /home/travis/build/kaliop-uk/ezmigrationbundle/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/FieldType/Date/SearchField.php line 32

It might be that a null value for the timestamp was previously ok, whereas it gets now converted to the string '@', which is not a valid representation. Maybe we could add a cast to int on that line 32 ?