Filtering ignore null values (#1129).

[wesdrasalves]

Description

A few sentences describing the overall goals of the pull request's commits.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
Currently the filtering will not accept null value, error when casting data to string. (# 1129)

What is the new behavior?
The filtering ignore null value now and no more error occurs.

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

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[amanmahajan7]

Thanks for the PR