fix(stateful table): enable empty string filtering in combo-box

[erzhan-temir-mamyrov]

Closes #3529

Summary

• Add special case filtering by empty string

Change List (commits, features, bugs, etc)

• Modify filter handlers in FilterHeaderRow to enable filtering by empty string
• Add unit test
• Update Storybook to reproduce the issue

Acceptance Test (how to verify the PR)

• Single-select filter
• Go to this story
• In the Knobs section enable Show empty string option in Select filter
• Reset current filters
• Select Unspecified in Select combo-box
• Verify that table has been filtered according to empty value in Select column

• Multi-select filter
• Same story
• Enable Show empty string option in Select filter
• Enable Allow multi-select in Select filter
• Reset current filters
• Select Unspecified in Select combo-box
• Verify that table has been filtered according to empty value in Select column
• Select second option in the multiselect (for example option-A)
• Verify that table has been filtered according to empty value in Select column

• Advanced filter
• Same story
• Enable Show empty string option in Select filter
• Enable Allow multi-select in Select filter
• Enable Enable advanced filters
• Reset auto-applied advanced filters
• Click on the filter icon
• In the Simple filters tab:
• Select Unspecified in Select combo-box
• Click on Apply filters
• Verify that table has been filtered according to empty value in Select column
• Select second option in the multiselect (for example option-A)
• Click on Apply filters
• Verify that table has been filtered according to empty value in Select column

Regression Test (how to make sure this PR doesn't break old functionality)

• tests here

Things to look for during review

• [ ] Make sure all references to iot or bx class prefix is using the prefix variable
• [ ] (React) All major areas have a data-testid attribute. New test ids should have test written to ensure they are not changed or removed.
• [ ] UI should be checked in RTL mode to ensure the proper handling of layout and text.
• [ ] All strings should be translatable.
• [ ] The code should pass a11y scans (The storybook a11y knob should show no violations). New components should have a11y test written.
• [ ] Unit test should be written and should have a coverage of 90% or higher in all areas.
• [ ] All components should be passing visual regression test. For new styles or components either a visual regression test should be written for all permutations or the base image updated.
• [ ] Changes or new components should either write new or update existing documentation.
• [ ] PR should link and close out an existing issue

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[jessieyan]

@erzhan-temir-mamyrov Just did some tests. Looks good to me.

One thing I noticed in Advanced filter, after I clicked on Apply filters button and open Advance filter again. The previous selections are cleared out. For example, previously I selected unspecified in Select. When open the advance filter again, it is not selected anymore. Is it as expected?

[erzhan-temir-mamyrov]

@jessieyan Thank you for bringing this up! This looks like a bug, I will try to fix it.