[MJAVADOC-767] javadoc creates invalid --patch-module statements

[hgschmie]

When using an exclude filter, it is possible that the plugin creates
javadoc --patch-module command line options, that do not actually have
a value.

This changes skips such empty entries.

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[slawekjaranowski]

please remember to add jira number in final commit

[hgschmie]

added integration test. To try out, remove this line: https://github.com/apache/maven-javadoc-plugin/pull/225/files#diff-3cc042c3116d0543702076b295213a5b4058ce2f0c342edf234dfd06fefacabfR44