<!--- This environment context section helps us quickly review your PR. 
      Please take a minute to fill-out this information. -->


## Environment
**Liquibase Version**: 4.4.0

**Liquibase Integration & Version**: maven

**Liquibase Extension(s) & Version**: 

**Database Vendor & Version**: postgres et al.

**Operating System Type & Version**: Windows 10 21H1

## Description
It seems in #964 a change in changelog validation was introduced, which now prohibits to set required string attributes to empty values. This is a problem, for example, if you want to use an addDefaultValue change to set an empty defaultValue for an existing column.

## Steps To Reproduce
The following minimal example will not pass validation in Liquibase 4.4.0, while it worked in Liquibase 4.3.5:


```xml
<changeSet author="me" id="1">
    <createTable tableName="MyTable">
        <column name="MyColumn" type="VARCHAR(64)"/>
    </createTable>
    <addDefaultValue columnName="MyColumn" tableName="MyTable" defaultValue=""/>
</changeSet>
```

## Actual Behavior
The above changeset will not pass changelog validation:


```
liquibase.exception.ValidationFailedException: Validation Failed:
    1 changes have validation failures
        defaultValue is empty, myChangelog.xml::1::me
```

## Expected/Desired Behavior
A changelog containing a changeset with an empty defaultValue attribute is still valid, like it was in Liquibase 4.3.5.


}
    @Override
    public Sql[] generateSql(AddDefaultValueStatement statement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        Object defaultValue = statement.getDefaultValue();
        return new Sql[] {
                new UnparsedSql("ALTER TABLE "
                        + database.escapeTableName(
                                statement.getCatalogName(),
                                statement.getSchemaName(),
                                statement.getTableName() )
                        + " ALTER COLUMN  "
                        + database.escapeColumnName(
                                statement.getCatalogName(),
                                statement.getSchemaName(),
                                statement.getTableName(),
                                statement.getColumnName()
                        ) + " SET DEFAULT "
                        + DataTypeFactory.getInstance()
                            .fromObject(defaultValue, database)
                            .objectToSql(defaultValue, database),
                    getAffectedColumn(statement) )
        };
    }
    protected Column getAffectedColumn(AddDefaultValueStatement statement) {
        return new Column()
                .setRelation(new Table().setName(statement.getTableName()).setSchema(new Schema(statement.getCatalogName(), statement.getSchemaName())))
                .setName(statement.getColumnName());
    }
}