DataTable filter should provide defaults for constraints and operator

[mrichar1]

Describe the bug

This is a 're-issue' of #1669 against PrimeVue 4.

When using Filter menu mode, if the filters object doesn't contain a constraints property, then when the match mode is selected from the UI the app crashes with the following error in the console:

_filters[this.field].constraints is undefined

Separately, if constraints is provided, but operator is unset, the UI doesn't display the Add Rule button, even if showFilterOperator is true.

Pull Request Link

No response

Reason for not contributing a PR

• Lack of time
• Unsure how to implement the fix/feature
• Difficulty understanding the codebase
• Other

Other Reason

No response

Reproducer

https://stackblitz.com/edit/primevue-4-vite-issue-template-3s53jroz

Environment

Ubuntu Linux 22.04 (but reproducible in sandbox)

Vue version

3.4.31

PrimeVue version

4.2.25

Node version

20.18.1

Browser(s)

No response

Steps to reproduce the behavior

1. Click on the filter icon - no errors in console and 'Add rule' button exists in UI.
2. Comment-out 'working' filter line and uncomment second one.
3. Click on the filter icon - error in console.
4. Comment out filter and uncomment 3rd one.
5. Click on filter icon - Add rule is missing from UI.

Expected behavior

There should be defaults set in the code such that not setting constraints or operator does not result in an error/unexpected behaviour.

There is detailed discussion of what these defaults might look like in the previous issue which I won't try to reproduce here.