Cannot specify empty string for configuration value

We have a MOJO class that involves several user-configured parameters. Some of these parameters have default values that we'd like to use in the event that the user doesn't specify a value for them; however, we'd also like the user to be able to explicitly specify that the parameter should not have a value. The approach we wanted to take was to specify default values in the parameter's annotation:

@Parameter(property = "foo.bar", defaultValue = "${project.scm.url}")
private String foo;

And then detect empty strings as the user's way of saying "I know there's a default value for this parameter but I'm sure that I'd actually like it to be empty anyways.":

<configuration>
  <foo></foo>
</configuration>

or

<configuration>
  <foo />
</configuration>

However, there's been some trouble as we haven't found a way yet to distinguish between an intentionally-empty string and one that hasn't been specified at all; the value for foo is always null regardless.

Is there a friendly and/or intuitive way for the user to pass an empty string as a parameter value?