fix: type null is not assignable to type IContent

[cjxe]

Description

1. I fixed type 'null' is not assignable to type 'IContent' error
2. I wrote a unit test in packages/main-library/src/__tests__/index.test.ts to test this change

Test

Test steps

1. Switch to this branch
2. Run nvm use 16.14.2 to switch to Node version 16.14.2 (install it using nvm if it's not installed yet)
3. Run yarn test to run the test files
4. Run yarn build to build the package

Expected results

• Test should pass without any errors
• The package builds without any errors

Further comments

The GH actions tests are failing because the tests are being run on the last commit (which is a broken commit).

To run it on the last commit, follow the steps:
1- reset “develop” branch to latest working commit (1dac19b)
2- sync your “develop” and “main” branch
3- merge this PR. It will be merged to the “develop” branch

Now, the tests should pass; because I ran GH actions on my bug reproduction environment

• My bug reproduction environment: https://github.com/cjxe/json-as-xlsx/pull/2

[LuisEnMarroquin]

cd Music/json-as-xlsx
gck develop
git reset --hard 1dac19b147ad767b9db9fa6be6feae22efce9305
gck main
gpl
git reset --hard origin/main
gpl
gck develop
git push origin develop --force
git merge main
gps