fix user deleting negation when it's the only character left

[Mikadifo]

Describe the issue/change

When the negative sign is the only character left in the input, the user can't delete it by pressing the Backspace key. This issue appeared with the validation that prevents the user from deleting a character when the caret is in middle of the prefix (when prefix is longer than 1 character).

Add CodeSandbox link to illustrate the issue (If applicable)

https://codesandbox.io/p/sandbox/multi-symbol-prefix-with-negative-qvqnfh
(I used the issue author codesandbox)

Describe specs for failing cases if this is an issue (If applicable)

Describe the changes proposed/implemented in this PR

To solve this issue, I added a validation that checks if the negative sign is the only character in the input. This goes before the partial prefix validation, that way the user can delete the negative sign when the prefix length is greater than 1.

Link Github issue if this PR solved an existing issue

Solves issue #882

Example usage (If applicable)

1. Create a NumericFormat element with prefix longer than 1 character and allowNegative set to true
2. Type multiple numbers including negative sign
3. Hit backspace and it now deletes the negative sign at the end

Please check which browsers were used for testing

• Chrome
• Chrome (Android)
• Safari (OSX)
• Safari (iOS)
• Firefox
• Firefox (Android)