Fixes CORE-1987 "mvn liquibase:diff" does not find any differences between databases

Fix empty schemaComparison
(cherry picked from commit efce4fe)
Fixed bug https://liquibase.jira.com/browse/CORE-1919 - liquibase throws Exception that DATABASECHANGELOGLOCK table is missing when dropFirst parameter is set to true
(cherry picked from commit b8401a8)
Merge remote-tracking branch 'origin/3.2.x' into 3.2.x