[NT-410] Handle optional paymentsource

[Scollaco]

📲 What

• This is a solution created by @justinswart that solves an issue on which Backing model fails deserializing on Staging because some fields of payment_source are not being returned on that environment.

🤔 Why

• This will allow us to test tasks related to backed projects on Staging until the data is returned normally by the server.

Before 🐛	After 🦋

[dusi]

Looks good.

From my conversation with @justinswart on Slack:

• I'd suggest that tryDecodePaymentSource is only used if we're in DEBUG mode and staging environment is set - just to be double sure this is not going to ship to App Store.
• Or if that seems too restrictive at least keep a compiler warning on this line to refer back to it soon ¯_(ツ)_/¯

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