[BUG] PR with no license headers didn't fail checks

[dbbaughe]

Describe the bug
This PR had some files with missing license headers. It should fail an automatic check instead of relying on human verification.

Expected behavior
Run ./gradlew build with a source file w/ no License Header and fail.

[dbbaughe]

Skips when there are no java files which is why task isn't running
https://github.com/elastic/elasticsearch/blob/master/buildSrc/src/main/groovy/org/elasticsearch/gradle/precommit/LicenseHeadersTask.groovy#L71