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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.


return this.change;
    }
    public ValidationErrors checkRequiredField(String requiredFieldName, Object value) {
        return checkRequiredField(requiredFieldName, value, null);
    }
    public ValidationErrors checkRequiredField(String requiredFieldName, Object value, String postfix) {
        String err = null;
        if (value == null) {
            err = requiredFieldName + " is required";
        } else if ((value instanceof Collection && ((Collection<?>) value).isEmpty())
                || (value instanceof Object[] && ((Object[]) value).length == 0)) {
            err = "No " + requiredFieldName + " defined";
        } else if (value instanceof String && StringUtil.trimToNull((String) value) == null) {
            err = requiredFieldName + " is empty";
        }
        if (err != null) {
            addError(err + (this.change == null ? "" : " for " + this.change)
                    + (postfix == null ? "" : postfix));
        }
        return this;
    }
    /**
     * <p>Checks if a field is forbidden in combination with a given Database (most often because that database
     * does not implement the features required by the field/value combination). If a "forbidden" use is detected,
     * a validation error is added to the current list of ValidationErrors.</p>
     * Note:
     * <ul>
     * <li>if value==null, the field is ALLOWED for all DBs</li>
     * <li>if the disallowedDatabases list does not at least contain 1 entry, the field is NOT allowed</li>
     * </ul>