Handle possible TypeError in the Cell._initCellBoot / _bootCellMirror APIs (SYN-7511)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.26%. Comparing base (be12b3c) to head (e659854).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3844 +/- ## ========================================== - Coverage 96.35% 96.26% -0.09% ========================================== Files 243 243 Lines 54036 54033 -3 ========================================== - Hits 52064 52016 -48 - Misses 1972 2017 +45 ``` | [Flag](https://app.codecov.io/gh/vertexproject/synapse/pull/3844/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject) | Coverage Δ | | |---|---|---| | [linux](https://app.codecov.io/gh/vertexproject/synapse/pull/3844/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject) | `96.26% <100.00%> (-0.01%)` | :arrow_down: | | [linux_replay](https://app.codecov.io/gh/vertexproject/synapse/pull/3844/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject#carryforward-flags-in-the-pull-request-comment) to find out more.

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