Bug 425227 - Orion Editor Content Assist Improvements
- Adapted js-tests/editor/contentAssist/testcases.js to new contentAssist filtering behavior which doesn't call computeProposals() for every model modification but rather does the filtering internally if possible.
- Also adapted tests to handle cases where extra unselectable proposals, such as separators, are returned by contentAssist.computeProposals().
- Added a couple more null checks in contentAssist.js code to handle cases where tests were passing in null or differently formatted proposals.

--Signed-off-by: Elijah El-Haddad <elijahe@ca.ibm.com>