fix: number field interactions

[williamw2]

Description

Allow the number field to be empty while typing. Original logic would prevent the deletion of all numbers in the field since the onchange event handler would ignore the change if the current value was NaN. This changes allows an empty string to be a valid condition. This also fixes a similar issue if a default value is specified for the number field.

Motivation & context

Fixes #3806

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

[EisenbergEffect]

Should we add a new test or two to assert the string deletion scenario stays fixed?

[williamw2]

Should we add a new test or two to assert the string deletion scenario stays fixed?

Added a test for the empty string scenario.