[Policy, no patch] Stop enforcing NULL for text config properties that do not need to differentiate from empty string

Problem/Motivation

Drupal core currently treats empty strings ('') in configuration as undesirable and encourages converting them to NULL. This convention was introduced inconsistently over time and has resulted in:

• unnecessary upgrade paths,
• brittle or non-functional hooks created solely to modify shipped configuration,
• extensive repetitive test changes,
• awkward developer experience in getters and validation,
• UI/API inconsistencies because forms only produce strings.

Recent issues (e.g., #3445150: Add validation constraints to core.entity_view_mode.*.* and #3533291: file_file_video_presave() attempts to update shipped configuration but doesn't work) demonstrate that enforcing '' → NULL provides little functional benefit but creates significant maintenance overhead. Several maintainers (@berdir, @longwave, @phenaproxima, @bbrala) agree that:

• NULL is not meaningfully better than an empty string for description or similar text fields,
• browsers and forms cannot operate on NULL, only strings,
• API getters already normalize NULL to '',
• distinguishing between “unset” and “empty” rarely matters for string config values,
• enforcing this rule creates churn and complexity without user-facing value.

In short: forcing empty strings to become NULL causes more problems than it solves.

Proposed Policy

For configuration properties that:

• store text,
• do not need to differentiate between “unset” and “empty”, and
• have no semantic requirement for NULL,

we should treat them as non-nullable strings.

The concrete implications:

1. Schema definitions for these properties should declare them as non-nullable string types.
2. Empty values should remain ''; no forced conversion to NULL.
3. If a module passes NULL internally, getters should normalize it to an empty string (e.g., $value ?? '').
4. No upgrade paths should be required solely to convert '' → NULL.

Benefits

• Eliminates unnecessary config-update hooks and deprecation notices,
• Removes upgrade paths that exist only for canonicalization,
• Reduces test churn and validation boilerplate,
• Provides consistent APIs that always return strings,
• Aligns behavior with how browsers and forms work,
• Reflects current maintainer consensus that forcing NULL adds no practical value.

This policy formalizes a more pragmatic, UI-aligned, and DX-friendly approach to text configuration values in Drupal core.