General coding issue: TypeScript compiler static check strictNullChecks to differentiate undefined vs. null

[danielweck]

See WIP PR:
#475
(broken build)

Example of potential fatal bug(crash):

https://github.com/readium/readium-desktop/blob/3ffe94ac5170b0454cc1f1c208fa348ea925cbcd/src/renderer/components/publication/Cover.tsx#L40-L46

...when this.props.publication.customCover is null, colors === undefined will not match, resulting in colors remaining null, and the rest of the code crashing. This could be detected if strictNullChecks was enabled in the TypeScript compiler settings (see the aforementioned WIP PR), but unfortunately this would currently break a huge amount of Thorium code, so we currently have to accept some degree of uncertainty (no static checks), and/or rely on unit-tests, and programmer's awareness (thus why I am filing this issue).