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

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

Shortcut the logic if differences.getDifference("columnNames") returns
null.
Merge pull request #197 from awallgren/master

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