Add support for handling nulls in pivot columns.

[deepankarsharma]

No description provided.

[texodus]

Thanks for the PR! The fix is much appreciated.

The tests seem to be failing to due a behavior change in floating point rounding - I assume this was not intentional and just an artifact of the order of operations for aggregates changing slightly, and is certainly inconsequential. A few notes for the future:

• There are alot of changes in this one commit - formatting changes, some references to a C++ test file not included in the PR, removal of the PSP_PARALLEL_FOR blocks, etc. Breaking this up into smaller commits/multiple PRs would assist me in merging these in a timely manner, as small commits are much easier to review.
• The tests need to pass - not just as a matter of principal, but because we have the repository configured to not allow merges on branches that fail on Travis CI. The puppeteer tests need documentation on how to use them certainly. You can review the modified screenshots/ directory to see the failures (see screenshot), and update these hashes automatically with the WRITE_TESTS=1 npm run test environment flag.

Otherwise, changes look good! I will fix the test hashes and merge out-of-band.

[deepankarsharma]

Thanks for the pointers on how to auto update the hashes. Will try to commit work in smaller increments in the future.