fix: Add morphologies branch to default config

[filimarc]

Describe the work done

The default configuration missed the morphologies branch, now should be ok.

[Helveg]

Is morphologies a required attribute? Could you give an example of an error this caused?

[filimarc]

It should not be a required attribute, but we found that when using a default configuration the __tree__() method ignore morphologies branch. So i thought it will be a quick fix for it...

[Helveg]

the tree() method ignore morphologies branch

The tree method intentionally leaves out any key that the user does not give. Can you clarify the problem some more? Here is what should happen:

• If you do not have morphologies in your configuration then __tree__ will not put it in the output either
• If you have 0 morphologies (morphologies = []) then __tree__ should put morphologies: [] in the output
• If you give morphologies, then __tree__ should output them.

Please show me with code examples if the current behaviour deviates from this description.

If you want to create a configuration that you can __tree__ to produce [] as output for morphologies (which is what your PR proposes to do by default, but I disagree with) then you can do the following:

from bsb import format_configuration_content, Configuration

format_configuration_content("json", Configuration.default(morphologies=[]))

Feel free to reopen this PR if you still feel like we should always add an empty morphologies array to the default configuration.