Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
/**
	 * Private method to validate the value of a property.
	 * 
	 * @param prop
	 *            definition of the property to validate
	 * @param value
	 *            the value to validate
	 * @return the value to store for the property
	 * @throws PropertyValueException
	 *             if the value is not valid
	 */
	private Object validateValue( ElementPropertyDefn prop, Object value )
			throws SemanticException
	{
		
		if ( value == null )
			return null;
		Object input = value;
		
		if ( value instanceof DesignElementHandle )
		{
			DesignElementHandle elementHandle = (DesignElementHandle) value;
			Module root = elementHandle.getModule( );
			input = ReferenceValueUtil.needTheNamespacePrefix( elementHandle
					.getElement( ), root, module );
		}
		Object retValue = null;
		try
		{
			retValue = prop.validateValue( module, input );