liquibase-core/src/main/java/liquibase/util/SmartMap.java
@Override
    public Object put(String key, Object value) {
        if (value == null) {
            return values.remove(key);
        } else {
            return values.put(key, value);
        }
    }
@Override
    public Object put(String key, Object value) {
        if (value == null) {
            return values.remove(key);
        } else {
            return values.put(key, value);
        }
    }
Like normal {@link Map#put(Object, Object)} operation, but if null is passed as "value" it removes the key/value from the map.