Pluck will error on null value

[mbonneau]

Pluck uses isset to check for the existence of a key. isset will not return true if the associated value is null which causes pluck to error.

This should use array_key_exists instead. (property_exists for objects)

Example:

\Rx\Observable::of([null])
    ->pluck(0)
    ->subscribe(
        fn($x) => printf("Next: %s\n", json_encode($x)),
        fn(\Throwable $e) => printf("Error: %s\n", $e->getMessage()),
        fn() => printf("Completed.\n")
    );

Outputs:

Error: Unable to pluck "0"

The expected output would be:

Next: null
Completed.

---

Here is the object version which exhibits the same behavior:

\Rx\Observable::of((object)['foo' => null])
    ->pluck('foo')
    ->subscribe(
        fn($x) => printf("Next: %s\n", json_encode($x)),
        fn(\Throwable $e) => printf("Error: %s\n", $e->getMessage()),
        fn() => printf("Completed.\n")
    );