Lib_context: factorize context binary creation

Context

The commit says "allow" context binary creation but actually it's already possible. This merge request is just a proposal to refactor the declaration of make_empty_context and make_empty_tree to facilitate the usage when the context is binary. I believe, we soru people, are the only users of this function.

Putting @igarnier as reviewer as you introduced the function make_empty_context :-).

Manually testing the MR

CI?

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
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