Convert null to undefined in the replace field of hints

[benchristel]

This fixes a Perseus JSON parser error observed in production. replace is
null in some assessment items.

Issue: none

Test plan:

yarn test

[github-actions]

Size Change: +7 B (0%)

Total Size: 872 kB

Filename	Size	Change
packages/perseus-core/dist/es/index.js	29.8 kB	+7 B (+0.02%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	39.5 kB
packages/keypad-context/dist/es/index.js	760 B
packages/kmath/dist/es/index.js	11.1 kB
packages/math-input/dist/es/index.js	78.2 kB
packages/math-input/dist/es/strings.js	1.79 kB
packages/perseus-editor/dist/es/index.js	276 kB
packages/perseus-linter/dist/es/index.js	22.8 kB
packages/perseus-score/dist/es/index.js	20.6 kB
packages/perseus/dist/es/index.js	367 kB
packages/perseus/dist/es/strings.js	6.74 kB
packages/pure-markdown/dist/es/index.js	4.14 kB
packages/simple-markdown/dist/es/index.js	13.1 kB

_{compressed-size-action}

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (55662a8) and published it to npm. You
can install it using the tag PR2267.

Example:

pnpm add @khanacademy/perseus@PR2267

If you are working in Khan Academy's webapp, you can run:

./dev/tools/bump_perseus_version.sh -t PR2267