Fix/workaround for https://liquibase.jira.com/browse/CORE-1532

Shortcut the logic if differences.getDifference("columnNames") returns
null.
(cherry picked from commit 3bef300)
Fix/workaround for https://liquibase.jira.com/browse/CORE-1532

Shortcut the logic if differences.getDifference("columnNames") returns
null.