MongoDB does not allow to create unique partial index in field that contains null
Please use the below query to create the unique partial index. The reason why your query work is, there may many fields with the value like {rss_id: null}

db.getCollection('noticias').createIndex(
   { rss_id: 1},
   { unique:true, partialFilterExpression: {rss_id: {$ne:null}}}
)


Or you can use sparse index option along with unique index as shown below

db.getCollection('noticias').createIndex( { rss_id: 1}, { sparse: true, unique: true })