Updates to delay functionality

[ecairol]

There are multiple plugins that are staying in the Auto-updates Disabled bucket. Reported for Brodo but is happening in other sites as well.

Changes in this Pull Request:

• In case of NULL, no-slug is changed to an empty string. Useful to validate later with empty()
• Because the slug could be empty, it was replaced with the plugin name in two places:
  • The database object that keeps track of everything.
  • The function get_installed_plugin_version which previously used slug in combination of dirname to get the version.
  • An example of name is woocommerce/woocommerce.php. This value is never an empty string or NULL.
• The function get_plugins() is executed only once when the class loads, and store and re-use it as an object property
• When calculating has_delay_passed(), if the slug is empty we will not be able to call Dotorg API. I saw plugins with empty slug stored both in GitHub and SVN/Trac, some with a changelog.txt and others didn't have explicit release versions, so I suspect it's nearly impossible to account for all of them. The safest case, IMO, is returning true for these cases where the slug is null.
• Similarly, if the current version is the last one, also return true
• The option_name was updated from plugin_update_delay to plugin_update_delay_2, to make sure we start from scratch (especially since we are now using plugin name instead of slug).

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Test

These changes were tested in Brodo Dev, and the number under Auto-updates Disabled is now 3, all justified.
https://brodo-development-1712936759.mystagingwebsite.com/wp-admin/plugins.php

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GitHub Issue:
https://github.com/a8cteam51/team51-dev-requests/issues/4479