Partial change to fix some warning if strictNullCheck is enabled for merge-tree package

[curtisman]

strictNullCheck for merge-tree is not enabled by default yet. Doing it in multiple PR to manage churn and change size.

Mostly simple change to typing to add undefined

Related to #2358

[msfluid-bot]

■ @fluidframework/base-host: No change

Metric Name	Baseline Size	Compare Size	Size Diff
main.js	166.8 KB	166.8 KB	■ No change
Total Size	166.8 KB	166.8 KB	■ No change

⯆ @fluid-example/bundle-size-tests: -124 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	144.54 KB	144.48 KB	⯆ -62 Bytes
odspDriver.js	192.74 KB	192.74 KB	■ No change
sharedString.js	158.59 KB	158.53 KB	⯆ -62 Bytes
Total Size	734.02 KB	733.9 KB	⯆ -124 Bytes

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Baseline commit: 0ec7ac7

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