Checkstyle enforcement of blank lines

Checkstyle has a rule (EmptyLineSeparator) we currently don't use to enforce the use of blank lines in various places. In order to reduce the noise in reviews of PR's, I like to start using it.

This will likely be accomplished via a series of PR's. Just adding the default EmptyLineSeparator to the checkstyle rules results in a THOUSANDS of violations which would be hard to review. I will likely enable it for one token type at a time, fix the violations on that particular token type, submit, add then next token type, etc... That should (hopefully) keep each PR down to something that can reasonably be reviewed.

Imported from Jira [BEAM-3640](https://issues.apache.org/jira/browse/BEAM-3640). Original Jira may contain additional context.
Reported by: dkulp.

Start by locating Apache Beam's Checkstyle configuration and reviewing the EmptyLineSeparator rule and its current violations. Enable one token type at a time, fix that token type's violations, and consider the work done when the focused change is reviewable without leaving violations for that token type.

Written by the indexing model from the issue text.