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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.


*
     * @param disallowedFieldName field whose value is checked
     * @param value               value that might be disallowed
     * @param database            database the object/value combination is checked against
     * @param disallowedDatabases a list of "forbidden" databases that do not allow this field/value combination
     */
    @SafeVarargs
    public final void checkDisallowedField(String disallowedFieldName, Object value, Database database,
                                           Class<? extends Database>... disallowedDatabases) {
        boolean isDisallowed = false;
        if ((disallowedDatabases == null) || (disallowedDatabases.length == 0)) {
            isDisallowed = true;
        } else {
            for (Class<? extends Database> databaseClass : disallowedDatabases) {
                if (databaseClass.isAssignableFrom(database.getClass())) {
                    isDisallowed = true;
                }
            }
        }
        if (isDisallowed && (value != null)) {
            addError(disallowedFieldName + " is not allowed on " + (database == null ? "unknown" : database.getShortName()));
        }
    }
    public ValidationErrors addError(String message) {
        errorMessages.add(message);
        return this;
    }
    public List<String> getErrorMessages() {
        return errorMessages;
    }