KeyFilter pValidateOnly does not accept Null values

[billfranklin]

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**Current behavior**
<!-- Describe how the bug manifests. -->
If you set a pKeyFilter="pnum" on a Reactive Form, and set the Validator.max(99.9999) (probably any value), the formcontrol has error of 'validatePattern' if the value is null or undefined.

**Expected behavior**
<!-- Describe what the behavior would be without the bug. -->
null and undefined should not cause a validatePattern error

PrimeNg Version: 5.2
Angular Version: 5.2.3

[AlejandroFlorin]

Same issue. But it is inconsistent. It happens with some controls but not others. Also happens with other filter types (int, num, etc.) I haven't found why yet.

[riorudo]

It seems that the default masks do not match/allow empty strings (https://github.com/primefaces/primeng/blob/master/src/app/components/keyfilter/keyfilter.ts).
In my case I use my own regex instead of "pint", I use /[\d]|^$/ to match empty strings as well.