build: Enable Checkstyle SingleSpaceSeparator check

[valfirst]

Change list

• Automate single space separator check, it will help to avoid cases like that: refactor: Migrate to new Selenium API for process management #2054 (comment)
• Fix all violations of the new rule

Types of changes

What types of changes are you proposing/introducing to Java client?
Put an x in the boxes that apply

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)