Only output identity for non-free (#1258)

Author: pamelafox

infra/core/search/search-services.bicep

@@ -62,4 +62,4 @@ resource search 'Microsoft.Search/searchServices@2021-04-01-preview' = {
 output id string = search.id
 output endpoint string = 'https://${name}.search.windows.net/'
 output name string = search.name
-output principalId string = search.identity.principalId
+output principalId string = (sku.name == 'free') ? '' : search.identity.principalId