fix: skip empty object write for untouched localized fields in mergeLocalizedData

[rachit367]

What?

Stop mergeLocalizedData from writing {} for a localized field that has no value in any locale and no incoming value.

In packages/payload/src/utilities/mergeLocalizedData.ts, the default case of the localized branch:

• Reads existingValue from the doc and only treats it as locale data when it is a plain object (not arrays, not nullish, not other primitives).
• Drops the previous else that unconditionally assigned existingValue (which had been defaulted to {}). The preservation path now only runs when there is real existing locale data.
• When newValue is not a valid object and there is no existing locale data, the field is left absent on the result instead of being written as {}.

Two unit tests added under the existing simple fields group:

• should not write an empty object when localized field has no value in any locale
• should preserve existing locale data on a localized field when new value is undefined

Why?

Fixes #16296.

When publishSpecificLocale is used on a document where a localized field has never been written in any locale, the merge path took docWithLocales[field.name] || {} and then fell through to result[field.name] = existingValue. That wrote {} to the DB for the untouched field, so a later read with locale: 'all' returned subtitle: {} instead of the field being absent.

The repro from the issue exercises this through _community integration tests; the unit-level behavior is the same and is now covered directly in mergeLocalizedData.spec.ts.

How?

Scope is limited to the default case of the localized branch in mergeLocalizedData. Behavior in the other branches (arrays, blocks, groups, tabs) is unchanged.

Walk-through of cases in the modified branch:

• Incoming locale object on a previously-empty field: still merges into a fresh {} and writes the new locales — unchanged.
• Incoming locale object on a field that already has other locales: still merges over the existing locales — unchanged.
• No incoming value, existing locales present: still preserved.
• No incoming value, no existing locales: previously wrote {}, now leaves the field absent.
• Child of a localized parent: still replaces with the new value — unchanged.

No other call sites needed to change. mergeLocalizedData always returns a plain object whose field.name may or may not be present; callers already handle the absent case the same way they would have handled undefined.