fix(plugin-detail): an empty array is not a cell value on the record page

.changeset/8474-empty-array-is-not-a-cell-value.md

@@ -0,0 +1,44 @@
+---
+'@object-ui/plugin-detail': minor
+---
+
+**An empty array is no longer a cell value on the record page (objectui#8474).**
+
+`hasCellValue` — THE definition of emptiness for `record:details` (objectui#8376,
+widened to the package by objectui#8394, extracted to `emptiness.ts` by
+objectui#8457) — opened with
+`if (value !== null && typeof value === 'object') return true;`. `typeof []` is
+`'object'`, so an **empty array was a value** and nothing further was asked of
+it.
+
+The object half's reasoning is sound and stays: an object value is handed to a
+type-aware renderer that knows how to draw it. But every example it reasons
+about is a *populated* object, and for `[]` the renderer has nothing to draw —
+`SelectCellRenderer` tests `value == null || value === ''`, which `[]` passes,
+then maps it over zero entries. The result was the blank cell the em-dash exists
+to prevent, produced by the function that exists to prevent it, plus the full
+objectui#8376 triple: the row escaped `emptyCount`, `canCopy` offered to copy it,
+and because `shouldAutoHideEmpty` needs only `filledCount > 0` a section whose
+one non-null value was `[]` armed auto-hide by itself and buried every genuinely
+empty row around it.
+
+**The change.** One arm inside the object branch:
+`if (Array.isArray(value) && value.length === 0) return false;`. All five readers
+of the shared authority get the corrected answer — `DetailSection`'s affordance,
+`emptyCount` and `canCopy`; `HeaderHighlight`'s highlight strip; `DetailView`'s
+summary chips; `HistoryTimeline`'s diff placeholder; `RecordMetaFooter`'s actor
+test.
+
+**What did NOT move, and why it is a measurement.** `{}` is still a value: on
+`json`, `object` and `location` fields it draws the literal `{}` through
+`JsonCellRenderer`, so there is no blank cell to fix. The shape that would have
+swept it in — `Object.keys(value).length === 0` — is also true of a `Date`, a
+populated `Map`, a populated `Set` and a getter-backed class instance, which
+would be a false-empty on values that render.
+
+**Declared cost.** A `json`-family field holding `[]` previously rendered the
+literal two-character text `[]`; it now draws the `No value` placeholder. That is
+intended and pinned.
+
+`RelatedList.isValueEmpty` is untouched: it already drew this line, and the
+shared authority moved toward it — never the reverse.