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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.


public ValidationErrors addWarning(String message) {
        warningMessages.add(message);
        return this;
    }
    public List<String> getWarningMessages() {
        return warningMessages;
    }
    public ValidationErrors addAll(ValidationErrors validationErrors) {
        if (validationErrors == null) {
            return this;
        }
        this.errorMessages.addAll(validationErrors.getErrorMessages());
        return this;
    }
    public void addAll(ValidationErrors validationErrors, ChangeSet changeSet) {
        for (String message : validationErrors.getErrorMessages()) {
            this.errorMessages.add(message + ", " + changeSet);
        }
        for (String message : validationErrors.getWarningMessages()) {
            this.warningMessages.add(message + ", " + changeSet);
        }
    }
    @Override
    public String toString() {
        if (getErrorMessages().isEmpty()) {
            return "No errors";
        }
        return StringUtil.join(getErrorMessages(), "; ");
    }
    @Override