Fixes a regression from PR 4982

After PR 4982, the rendering of the first two pages of http://www.openmagazin.cz/pdf/2011/openMagazin-2011-04.pdf (from issue 215) no longer completes.

The issue is that we cannot have `args === null` in `PartialEvaluator_buildPath`, but *must* use an empty array instead.

In this patch I've also moved the `argsLength` variable definition in `EvaluatorPreprocessor_read`, to make sure that it's always defined.