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## 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.


package liquibase.sqlgenerator.core;
import liquibase.database.Database;
import liquibase.database.core.HsqlDatabase;
import liquibase.datatype.DataTypeFactory;
import liquibase.datatype.LiquibaseDataType;
import liquibase.datatype.core.BooleanType;
import liquibase.datatype.core.CharType;
import liquibase.exception.ValidationErrors;
import liquibase.sql.Sql;
import liquibase.sql.UnparsedSql;
import liquibase.sqlgenerator.SqlGeneratorChain;
import liquibase.statement.DatabaseFunction;
import liquibase.statement.SequenceNextValueFunction;
import liquibase.statement.core.AddDefaultValueStatement;
import liquibase.structure.core.Column;
import liquibase.structure.core.Schema;
import liquibase.structure.core.Table;
public class AddDefaultValueGenerator extends AbstractSqlGenerator<AddDefaultValueStatement> {
    @Override
    public ValidationErrors validate(AddDefaultValueStatement addDefaultValueStatement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        Object defaultValue = addDefaultValueStatement.getDefaultValue();
        ValidationErrors validationErrors = new ValidationErrors();
        validationErrors.checkRequiredField("defaultValue", defaultValue);
        validationErrors.checkRequiredField("columnName", addDefaultValueStatement.getColumnName());