Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/DataSetParameterHandle.java
public boolean isOptional( )
	{
		return ( (Boolean) getProperty( DataSetParameter.IS_OPTIONAL_MEMBER ) )
				.booleanValue( );
	}
	/**
	 * Sets whether the parameter is optional.
	 * 
	 * @param value
	 *            the value to set
	 */
	public void setIsOptional( boolean value )
	{
		setPropertySilently( DataSetParameter.IS_OPTIONAL_MEMBER, Boolean
				.valueOf( value ) );
	}
	/**
	 * Sets the default value of the input parameter.
	 * 
	 * @param expr
	 *            the default value
	 */
	public void setDefaultValue( String expr )
	{
		setPropertySilently( DataSetParameter.DEFAULT_VALUE_MEMBER, expr );
	}
	/**
	 * Gets the default value of the input parameter.
	 * 
	 * @return the default value