Description of PR:

Convert null or undefined cell values to an empty string

Fixes #1502

PR checklist:

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fixes #xxxx[,#xxxx], where "xxxx" is the issue number)

If new features/enhancement/fixes are added or changed:

• Includes documentation updates
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (does it affect screen reader users or keyboard only users?)

If adding a new feature, or changing the functionality of an existing feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

PR titles should following the Conventional Commits naming convention