The spec for javax.lang.model.element.PackageElement.getQualifiedName() says that
Returns: the fully qualified name of this package, or the empty string if this is the unnamed package

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.Elements;
import java.util.Set;

@SupportedAnnotationTypes("*")
public class Test extends AbstractProcessor {
    public boolean process(Set<? extends TypeElement> tes, RoundEnvironment renv) {
        if(renv.processingOver()) return true;

        Elements elements = processingEnv.getElementUtils();
        System.out.println("empty package is '" +
                elements.getPackageElement("").getQualifiedName() + "'");
        return true;
    }
}

F:\projects\jck>Z:\lnk\re\jdk\6.0\latest\binaries\windows-i586\bin\javac.exe -cp classes -processor Test src/Test.java
empty package is 'unnamed package'