Provide a default value for scheme_allow_list

Problem/Motivation

Pathologic is using scheme_allow_list to avoid processing some urls (previously known as scheme_whitelist and pathologic_schema_whitelist in D7). Under some circumstances the setting can be null (for example when it is config ignored). This has been already reported and mitigated at #3216344: \Drupal::config returns null if the config isn't set. In that case, there's no error, but pathologic does not replace absolute urls.

Steps to reproduce

Import any pathologic.settings.yml without the scheme_allow_list. Create some content with a formatted text field and the pathologic filter applied in that text format. Enter in the formatted text field a link with an absolute url with a base_url included in the pathologic configuration (the base_url of the link should be different to the current site/environment base_url). The base_url of the link won't be replaced as expected.

Proposed resolution

Use ['http', 'https', 'files', 'internal'] as fallback instead of empty array, as it was in D7: #1834308: Configurable white-list for $parts['scheme'], which will allow schema links to be passed to hook_pathologic_alter()

Remaining tasks

- Patch
- Review
- Test

User interface changes

None

API changes

None

Data model changes

None