Main graph fixes

[RobertJoonas]

Changes

Fixes three bugs:

1. Fix main graph crashing when hovering on a time bucket that includes only comparison data, and then changing the query period with a keyboard shortcut. Before (master staging) -> After (this review env). Note that the actual crash can be difficult to reproduce. I assume it's timing-related, but not fully sure why. In any case, in the page console you can clearly see master throwing errors, but review env not.
2. Fix displaying tooltips for original-range-only time buckets when comparison range is shorter. Before (master staging) -> After (this review env)
3. Do not link to __blank__ periods when clicking on a time bucket that includes only comparison data (use the same before/after links as 1) to reproduce)

Currently, if the comparison period is longer, the backend fills the "overflow" comparison period with empty metrics, which is not correct, as there is actually data in those buckets.

Hovering anywhere in that right hand side area will display lots of console errors because the graph-tooltip.js module is currently not accounting for missing values. This PR starts doing that to avoid the crash, and on top of that, not even trying to render a tooltip. This will be fixed properly with #6152 and #6159.

Note: I tried to add an E2E test for both of these fixes, but testing the canvas is currently insanely difficult. TODO: write E2E tests once #6159 has rebuilt line-graph as SVG.

Tests

• Automated tests have been added

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• This PR does not change the UI

[github-actions]

Preview environment👷🏼‍♀️🏗️
PR-6169