fix(feature flags): Guard for None values when comparing person Properties

[Phanatic]

Problem

Porting over the fix from posthog-node to posthog-python : PostHog/posthog-js-lite#249

The current behavior of stringifying None values to compare with operators can lead to invalid results in local evaluation.

This causes local evaluation to return true, while the /decide API does the right thing and returns false for undefined values in these properties.

Customer reported that local evaluation always evaluates a condition to true, when it should evaluate to false.
The conditions in question are :

[
    {
        "key": "latestBuildVersion",
        "type": "person",
        "value": ".+",
        "operator": "regex"
    },
    {
        "key": "latestBuildVersionMajor",
        "type": "person",
        "value": "23",
        "operator": "gt"
    },
    {
        "key": "latestBuildVersionMinor",
        "type": "person",
        "value": "31",
        "operator": "gt"
    },
    {
        "key": "latestBuildVersionPatch",
        "type": "person",
        "value": "0",
        "operator": "gt"
    }
]

And the person properties passed into local evaluation are :

{
    "personProperties": {
        "latestBuildVersion": None,
        "latestBuildVersionMajor": None,
        "latestBuildVersionMinor": None,
        "latestBuildVersionPatch": None
    }
}

Since we stringify all of the None values, the above call returns true when it should return false because the property values are undefined and not the string None.

Changes

Add a NONE_VALUES_ALLOWED_OPERATORS array of operators that allow NONE values which is just is_not right now . Any other operator will fail if the overrideValue is NONE

[dmarticus]

Will this need a version bump as well?