Guard against falsy render param value for 'funding.allowed'

[dechov]

Fixes https://github.com/paypal/paypal-checkout/issues/1006

Guards against falsy values (e.g. null) passed as the funding.allowed parameter in the Button render code (similar to how allowed is checked for 16 lines above), to avoid breaking the render.

[mcntsh]

Thanks @dechov for your PR. This bug is a P1 impacting several merchants so we went ahead and filed our own which addresses a couple of other use-cases:

1. All non-array values (not just falsy)
2. Disallowed list as well as Allowed.

We really appreciate your report and your quick PR. This fix will be out in the next hour.