When you say "", do you mean it's a blank field? Null? Or was that a space?
Elasticsearch by default treats blank and null values as if they don't exist, so nothing is indexed.
You can use null_value as a way to index placeholders for null values: https://www.elastic.co/guide/en/elasticsearch/reference/current/null-value.html. Or when running the terms aggregation, you can specify a missing parameter which defines a placeholder to use when fields are missing/empty/null: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-terms-aggregation.html#_missing_value_12
Not quite the same, but related docs on null/missing/exist here: https://www.elastic.co/guide/en/elasticsearch/guide/current/_dealing_with_null_values.html