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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.


validationErrors.checkRequiredField("tableName", addDefaultValueStatement.getTableName());
        if (!database.supportsSequences() && (defaultValue instanceof SequenceNextValueFunction)) {
            validationErrors.addError("Database "+database.getShortName()+" does not support sequences");
        }
        if (database instanceof HsqlDatabase) {
            if (defaultValue instanceof SequenceNextValueFunction) {
                validationErrors.addError("Database " + database.getShortName() + " does not support adding sequence-based default values");
            } else if ((defaultValue instanceof DatabaseFunction) && !HsqlDatabase.supportsDefaultValueComputed
                (addDefaultValueStatement.getColumnDataType(), defaultValue.toString())) {
                validationErrors.addError("Database " + database.getShortName() + " does not support adding function-based default values");
            }
        }
        String columnDataType = addDefaultValueStatement.getColumnDataType();
        if (columnDataType != null) {
            LiquibaseDataType dataType = DataTypeFactory.getInstance().fromDescription(columnDataType, database);
            boolean typeMismatch = false;
            if (dataType instanceof BooleanType) {
                if (!(defaultValue instanceof Boolean) && !(defaultValue instanceof DatabaseFunction)) {
                    typeMismatch = true;
                }
            } else if (dataType instanceof CharType) {
                if (!(defaultValue instanceof String) && !(defaultValue instanceof DatabaseFunction)) {
                    typeMismatch = true;
                }
            }
            if (typeMismatch) {
                validationErrors.addError("Default value of "+defaultValue+" does not match defined type of "+columnDataType);
            }
        }
        return validationErrors;