Filter by null and not null

* Add cases/constants to handle null/not null to emscripten.cpp,
  base.cpp/.h, scalar.cpp, and python.h
* Fix table.cpp so that null values are not skipped when filtering by null
* Alter _is_valid_filter in emscripten.cpp with a special case for
  null/not null so it doesnt reject those filters incorrectly
* Refactor constants.js to allow for accessing filter operators in a
  non magic string way
* Add special condition to row.js filter update so that the operand
  field is hidden when selecting null/not null filter
* Add special condition to perspective.js is_valid_filter so null/not
  null is always valid (operand is irrelevant in this case)
* Add Specs

Remove NaN filtering and related feature flags

* Retain the "true" behavior for nan handling where applicable