fix(api): reject empty label on PATCH /stamps/{batch_id}

[martinconic]

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Description

PATCH /stamps/{batch_id} accepts an empty or missing label and returns 200, silently blanking the issuer's label — even though the OpenAPI spec marks label as required.

Fixing #5510

Fix

Reject an empty/whitespace-only label with 400 "label is required". This makes the handler match the spec (which already requires label), so no OpenAPI change is needed. The happy path ({"label":"name"}) is unchanged.

Open API Spec Version Changes (if applicable)

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