_mapping ignores allow_no_indices parameter

[tombousso]

$ curl "localhost:9200/ababababababab*/_mapping"
{"error":{"root_cause":[{"type":"index_not_found_exception","reason":"no such index","index_uuid":"_na_","index":"ababababababab*"}],"type":"index_not_found_exception","reason":"no such index","index_uuid":"_na_","index":"ababababababab*"},"status":404}

$ curl "localhost:9200/ababababababab*/_mapping?allow_no_indices=true"
{"error":{"root_cause":[{"type":"index_not_found_exception","reason":"no such index","index_uuid":"_na_","index":"ababababababab*"}],"type":"index_not_found_exception","reason":"no such index","index_uuid":"_na_","index":"ababababababab*"},"status":404}

$ curl "localhost:9200/ababababababab*/_mapping?allow_no_indices=false"
{"error":{"root_cause":[{"type":"index_not_found_exception","reason":"no such index","resource.type":"index_or_alias","resource.id":"ababababababab*","index_uuid":"_na_","index":"ababababababab*"}],"type":"index_not_found_exception","reason":"no such index","resource.type":"index_or_alias","resource.id":"ababababababab*","index_uuid":"_na_","index":"ababababababab*"},"status":404}

Tested on Elastic versions 6.3.0 and 6.1.3

Expected behavior:
localhost:9200/ababababababab*/_mapping?allow_no_indices=true returns {}

[elasticmachine]

Pinging @elastic/es-search-aggs