Empty dump management

[sveetch]

Is your feature request related to a problem? Please describe.

Currently, when a dump does not have any data, the dump is still created but empty. Then diskette_load try to load it like the other, it won't fail but output a warning about empty data and that it is possible corrupted or invalid.

Describe the solution you'd like

The empty dump itself is harmless since it does not fail and is around 6byte size. However it can be troublesome to users to see warning when loading dumps with empty data.

• Empty dump should always be dumped and archived, i think it would be a wrong idea to ignore them.
• At least they should not be loaded, we could assert about a size (something like <10bytes) assumed to be empty and just mark it as ignored;
• The assumed "empty dump" size should be either defined from setting (so in some case user can level it up or down for some needing) or the "ignore empty" feature should be optional;

[sveetch]

LoaddataSerializerAbstract.call can know the size of dump but it may not be the better place, Loader.deploy_datas would have been better for filtering.

For how to know if a dump is empty:

• Dump file should be less or equal to the minimal size setting;
• If size match, open the file and parse it with json
• If contained list is empty, it is to be ignored;
• If JSON fail or file does not exists, let it flow to the django call command so it will raise a proper error;

[sveetch]

Done in v0.3.5-pre.2 except it does not "let if flow to the django call command", so if file does not exists, this will fail with an exception.