`ext-rrid` field type fails validation when optional and left empty

[ree-gupta]

When using the ext-rrid input type with requiredValue: false, leaving the field empty causes a validation error:

{
  "message": "object instance has properties which are not allowed by the schema: ['@value']",
  "location": "/strain",
  "additionalInfo": {
    "schemaFile": "#",
    "schemaPointer": "/properties/strain"
  }
}

Expected behavior:

An optional ext-rrid field left empty should validate successfully.

Actual behavior:

CEDAR outputs {"@value": null} for empty ext-rrid fields, but the generated schema only allows @type, rdfs:label, and @id properties. With additionalProperties: false, this causes validation failure.

Use case:

In our neurophysiology metadata template, strain uses ext-rrid to capture RRID identifiers for animal strains. However, strain is not applicable for all species (e.g., non-human primates, human subjects), so the field must be optional.

Is ext-rrid designed to be used only with required fields? If so, could this be documented? If not, could the generated schema include @value to handle the empty case?

PS: The RRID lookup returns mixed-quality results - qualified RRIDs (e.g., IMSR_JAX:000664) resolve correctly, but some unqualified entries (e.g., RRID:3028467) do not resolve. This may be upstream data quality, but users have no way to distinguish reliable entries in the UI.