Fixes NPE in JavadocLikePackageFilter when matching against classes in the default (empty) package

[ntherning]

Without this patch I get an NPE like this:

Execution of JApiCmp failed: null
java.lang.NullPointerException
    at java.util.regex.Matcher.getTextLength(Matcher.java:1283)
    at java.util.regex.Matcher.reset(Matcher.java:309)
    at java.util.regex.Matcher.<init>(Matcher.java:229)
    at java.util.regex.Pattern.matcher(Pattern.java:1093)
    at japicmp.filter.JavadocLikePackageFilter.matches(JavadocLikePackageFilter.java:29)
    at japicmp.filter.Filters.includeClass(Filters.java:63)
    at japicmp.cmp.JarArchiveComparator.applyFilter(JarArchiveComparator.java:190)
    at japicmp.cmp.JarArchiveComparator.compareClassLists(JarArchiveComparator.java:171)
    at japicmp.cmp.JarArchiveComparator.createAndCompareClassLists(JarArchiveComparator.java:153)
    at japicmp.cmp.JarArchiveComparator.compare(JarArchiveComparator.java:67)
    at japicmp.cli.JApiCli$Compare.run(JApiCli.java:77)
    at japicmp.JApiCmp.main(JApiCmp.java:16)

when invoking japicmp with an include pattern and the JAR files contain classes in the default package.

[siom79]

Merged pull request to develop branch.