Set slug default to null instead of empty string

[Copilot]

When creating or releasing documents without explicitly providing a slug, __rerum.slug was being set to "" instead of null. This created ambiguity between "no slug" and "empty string slug".

Changes

• controllers/crud.js: Initialize slug to null instead of ""
• controllers/release.js: Initialize slug to null instead of ""

The slug is only assigned a string value when the Slug header is explicitly provided. Otherwise, documents now have __rerum.slug: null, clarifying that no custom slug was set.

// Before
let slug = ""
if(req.get("Slug")){
    slug = slug_json.slug_id
}
rerumProp.__rerum.slug = slug  // "" when no Slug header

// After  
let slug = null
if(req.get("Slug")){
    slug = slug_json.slug_id
}
rerumProp.__rerum.slug = slug  // null when no Slug header

Existing MongoDB queries using {"$or":[{"_id": id}, {"__rerum.slug": id}]} continue to work correctly.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Prevent slug from ever being empty</issue_title>
<issue_description>Two ways to call a document: _id or __rerum.slug both are always unique. However, if the call is not specifically creating or updating a slug, it should not be sent. The default ought to be null not "".</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Prevent slug from ever being empty #231

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[thehabes]

Yes that is best