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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.exception;
import liquibase.Scope;
import liquibase.change.Change;
import liquibase.change.ChangeFactory;
import liquibase.changelog.ChangeSet;
import liquibase.database.Database;
import liquibase.util.StringUtil;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
public class ValidationErrors {
    protected List<String> errorMessages = new ArrayList<>();
    protected List<String> warningMessages = new ArrayList<>();
    protected String change = null;
    public boolean hasErrors() {
        return !errorMessages.isEmpty();
    }
    public ValidationErrors() {
    }
    public ValidationErrors(String change) {
        this.change = change;
    }
    public ValidationErrors(Change change) {
        this.change = Scope.getCurrentScope().getSingleton(ChangeFactory.class).getChangeMetaData(change).getName();
    }
    public String getChangeName() {