On a parameter defined like this:

<param type="string" id="my.param">
    <label>My param</label>
    <description>My param</description>
    <default-value>Default value</default-value>
    <category>My category</category>
    <group>My group</group>
</param>

If my configuration is not complete, the CMS is able to start because there is a default value and it is not mandatory.
So I call the method Config.getInstance("my.param", true, null), and I get null. I should get the default value. If I set a default value as the third parameter, I get this value instead of the real default value.