public Optional<String> getType(Set<String> streamIds, String field) {
        final Map<String, Set<String>> allFieldTypes = this.get(streamIds);
        final Set<String> fieldTypes = allFieldTypes.get(field);
        return typeFromFieldType(fieldTypes);
    }
@Test
    void returnsEmptyOptionalIfStreamsAreEmpty() {
        final Pair<IndexFieldTypesService, StreamService> services = mockServices();
        final FieldTypesLookup lookup = new FieldTypesLookup(services.getLeft(), services.getRight());
        final Optional<String> result = lookup.getType(Collections.emptySet(), "somefield");
        assertThat(result).isEmpty();
    }