I agree with Noordsestern, this is not natively supported.
Embedded arguments do not support default values or variable number of arguments like normal arguments do. If such functionality is needed, normal arguments should be used instead.
However, it is possible using the following trick: do not put space between the argument and the previous or following word. This way you avoid having two consecutive spaces when calling the keyword without value. You are actually calling it with the “empty string” value.
You also have to “implement” the default value using a condtional statement.
Note that you end up with a leading or trailing white space, so you might want to trim it with Strip String.