Fix regexp when filter by root category id

[mhhansen]

Description

At present, if you query the category tree requesting the Root Category (id = 1), query returns empty.
This is caused by the regular expression that filters the catalog_category_entity.path column table, which includes for all cases, a slash (/) at the beginning.
In the case of the Root Category, there's no slash at the beginning.

This has been tested for this scenarios:

• A fresh 2.3-alpha install, with no content created prior or after install.
• A fresh 2.3-alpha install, with other Root Category created post install.
• A fresh 2.3-alpha install, with sample-data created after installation.

For all cases, in the catalog_category_entity.path column table, categories are always created as a 1/ children.

Fixed Issues (if relevant)

Fixed issue: No issue created to this PR.
Issue related: Category tree depth calculation issue #100

Manual testing scenarios

To be able to test this PR, you have to first apply the PR #102, which fixes the category deph children param.

1. Query the category(id: 1), to get the entire Category
2. Expected result: get the Root Category with all the children.
3. Actual result:

Query the Root Category

{
category(id: 1) {
    name
    path
    children_count
    children {
      id
      name
      level
      children {
        id
        name
      }
    }
  }
}

Expected result

Actual result

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[naydav]

Looks like we need to use \Magento\Catalog\Model\Category::TREE_ROOT_ID instead of 1

[mhhansen]

that's exactly what I didn't found when I did that fix..
awesome! I will be pushing the update

[mhhansen]

@naydav welcome back!
I remember we talk that this one would require to break the class down (or something like that),
to avoid coupling 13 deps in the CategoryTree. ping me at anytime!

[magento-engcom-team]

@mhhansen thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.