Fix and unmute ExponentialHistogramBlockTests.testComponentAccess.

[JonasKunz]

Fixes #138399.
Looks like in #138349 this test slipped through due to randomization.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[felixbarny]

The test fix LGTM.

Looks like in #138349 this test slipped through due to randomization.

Is that because the case of the histogram having a value count of 0 is happening very rarely in the randomization? If so, I'm wondering if it would make sense to increase the probability of that branch, or to ensure both branches are hit on every test execution.

[JonasKunz]

Is that because the case of the histogram having a value count of 0 is happening very rarely in the randomization?

No, I was just extremely lucky (or unlucky..).
The test generates a random block of length 1-100. Each histogram has roughly a 25% chance of being empty.

I noticed another test failure from #138349, which I sneaked a fix and test improvement for into this PR aswell.