Revert "Javascript: support null pointers."

This reverts commit 11963788e0d008e1c6e997577086cd8dd584c03f.
Revert "JavaScriptCore: ConvertPtr should allow JavaScript null to be a valid value."

This reverts commit 05ed0325dc4bd9ddc126d3dacc6b38686f7382cf.
Javascript: Use RUNTOOL for running tests.