about:telemetry rendering breaks when some optional ping sections are not present

This fixes about:telemetry rendering for sections that are not always present, e.g. when extended Telemetry recording is disabled.

Comment on attachment 8608048 [details] [diff] [review] Make about:telemetry work properly for optional ping sections Approval Request Comment [Feature/regressing bug #]: Unified Telemetry, https://wiki.mozilla.org/Unified_Telemetry This is part of the first (main) batch of uplifts to 40 to enable shipping on that train, see bug 1120356, comment 2. [User impact if declined]: Data & measurement insight projects delayed or blocked with direct impact on projects depending on this. [Describe test coverage new/current, TreeHerder]: We have good automation coverage of the feature. We also had manual tests of the main tasks as well as confirmation of correct behavior on Nightly for the patches here. [Risks and why]: Low-risk - these patches are rather isolated to Telemetry and have been on Nightly for a while with no bad reports. We intend to track on-going data quality and other issues during the 40 aurora & beta and flip the new behavior off when it doesn't meet the requirements. [String/UUID change made/needed]: The only string changes were to the about:telemetry page. We decided that we can live with missing translations on that page for a cycle as that page is not exactly user-facing.

Comment on attachment 8608048 [details] [diff] [review] Make about:telemetry work properly for optional ping sections Unified telemetry is an important new feature. It is blocking some other projects. Taking it.