[APM] user can set an empty value for UI indices

[dgieselaar]

When the user is saving the UI indices settings, an empty value can be saved by filling in a value, and then removing it. This could break a query that uses the UI indices, because an empty index name leads to the following exception (string_index_out_of_bounds_exception):

server    log   [16:54:12.688] [error][http] { Error: [security_exception] action [indices:data/read/search] is unauthorized for user [admin]
    at respond (/Users/dariogieselaar/dev/kibana/node_modules/elasticsearch/src/lib/transport.js:349:15)
    at checkRespForFailure (/Users/dariogieselaar/dev/kibana/node_modules/elasticsearch/src/lib/transport.js:306:7)
    at HttpConnector.<anonymous> (/Users/dariogieselaar/dev/kibana/node_modules/elasticsearch/src/lib/connectors/http.js:173:7)
    at IncomingMessage.wrapper (/Users/dariogieselaar/dev/kibana/node_modules/elasticsearch/node_modules/lodash/lodash.js:4929:19)
    at IncomingMessage.elasticAPMCallbackWrapper (/Users/dariogieselaar/dev/kibana/node_modules/elastic-apm-node/lib/instrumentation/index.js:309:27)
    at IncomingMessage.emit (events.js:194:15)
    at endReadableNT (_stream_readable.js:1103:12)
    at process._tickCallback (internal/process/next_tick.js:63:19)
  status: 403,
  displayName: 'AuthorizationException',
  message:
   '[security_exception] action [indices:data/read/search] is unauthorized for user [admin]',
  path: '/apm-*%2Capm-*%2C%2Capm-*/_search',
  query: { terminate_after: 1, ignore_throttled: true },
  body:
   { error:
      { root_cause: [Array],
        type: 'security_exception',
        reason:
         'action [indices:data/read/search] is unauthorized for user [admin]',
        caused_by: [Object] },
     status: 403 },
  statusCode: 403,
  response:
   '{"error":{"root_cause":[{"type":"security_exception","reason":"action [indices:data/read/search] is unauthorized for user [admin]"}],"type":"security_exception","reason":"action [indices:data/read/search] is unauthorized for user [admin]","caused_by":{"type":"string_index_out_of_bounds_exception","reason":"String index out of range: 0"}},"status":403}',
  toString: [Function],
  toJSON: [Function] }

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)