[FIXED JENKINS-16719] Combobox with relative dependencies does not work

* core/src/main/resources/lib/form/combobox/combobox.js
  Increase index to 100, so that parent form elements can be initialized before
  parameters in fillDependsOn are extracted.

* test/src/test/java/lib/form/ComboBoxTest.java
* test/src/test/resources/lib/form/ComboBoxTest/CompoundField/config.jelly
* test/src/test/resources/lib/form/ComboBoxTest/CompoundFieldComboBoxBuilder/config.jelly
  New test to confirm null is not passed for relative dependent arguments