UI: Distinguish null vs empty string for feature values

[talissoncosta]

Description

The UI renders both null and "" (empty string) feature values identically — as a blank field. There's no way to visually distinguish a flag with no value (null) from one explicitly set to an empty string.

This causes:

• Confusion when reviewing flag values
• Misleading diffs in /compare (a null vs "" change shows as no visible difference)
• Uncertainty about how defaults should be treated

Context

PR #7146 made the UI "clear value" flow set values back to null instead of "", which removes one source of inconsistency for new edits. However:

• Historical data still holds "" values
• "" can still be set via the API or SDKs (not only the UI clear flow)
• The visual distinction problem remains regardless of how the value was created

Proposed Solution

Show an explicit placeholder when a value is null, using muted/italic text such as (no value) or (not set).

This avoids exposing the literal word null to non-technical users while clearly differentiating null from "".

Examples

• Value is null → display (no value) in muted italic
• Value is "" → display empty input as today (or with quotes around it for extra clarity)
• Value is "hello" → display hello as today

Where this appears

• Feature value cells in the features list
• Environment comparison view (/compare)
• Segment overrides
• Any other place a feature value is rendered

Reported by

Customer feedback.