[TEST] Wait for yellow since some shards might not be started

In this test we only index a handful of docs so if we have more shards
than docs we might fail on the `assertSearchResult` since not all shards
are started but results are just fine.
[Query] QueryParser can return null from a query

This causes a NPE since XContentStructure checks if the query is null
and takes this as the condition to parse from the byte source which is
actually null in that case.

Closes #6722
[TEST] assert search results before comparing the results - shards / indices might be missing
[TEST] use existing client that is already in the cluster to wait for joining node