More partial fixes for enabling strictNullCheck.

[curtisman]

No description provided.

[msfluid-bot]

■ @fluidframework/base-host: No change

Metric Name	Baseline Size	Compare Size	Size Diff
main.js	167.09 KB	167.09 KB	■ No change
Total Size	167.09 KB	167.09 KB	■ No change

⯆ @fluid-example/bundle-size-tests: -12 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
container.js	192.26 KB	192.26 KB	■ No change
map.js	45.9 KB	45.9 KB	■ No change
matrix.js	144.12 KB	144.11 KB	⯆ -6 Bytes
odspDriver.js	192.74 KB	192.74 KB	■ No change
sharedString.js	158.17 KB	158.17 KB	⯆ -6 Bytes
Total Size	733.18 KB	733.17 KB	⯆ -12 Bytes

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Baseline commit: 693c752

Generated by 🚫 dangerJS against fe09774

[tylerbutler]

[tylerbutler]

The double-bang was new syntax for me. :) https://medium.com/better-programming/javascript-bang-bang-i-shot-you-down-use-of-double-bangs-in-javascript-7c9d94446054

[curtisman]

Related to #2358