Cache context not applied for blocks without content

Problem/Motivation

When writing a test for #2524082: Config overrides should provide cacheability metadata → it was discovered that cache contexts are not bubbling up correctly for blocks that do not have any content. This is maybe a relic from Drupal 7 where blocks without content were not rendered. However in Drupal 8 the titles are rendered so it is important to bubble up the cacheability metadata.

To replicate this, check the CacheabilityMetadataConfigOverrideIntegrationTest from #2524082: Config overrides should provide cacheability metadata → - a line in the setUp() of that test explicitly adds some content to the test block. If that line is removed the cacheability metadata is not communicated any more and the test fails.

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