[BUG] Can't refine union type in tuple

[pranaygp]

Minimal repro:
https://flow.org/try/#0PTAEAEDMBsHsHcBQAXAngBwKagLIENUAjTAHgBUA+UAXlAG0A7AV2mgBpQyBdUAH3oCiAJyGwhHZqy6JomZKFQAuXAWIlmAW2JCqtRi3agAjNMQBjWAwDO8uphEcAlj1qpEjyKAAU9oQEpQAG9EUFAQel8nF0ERMQkDaVCLa1hZADo4AHMfETSNTCsrPEzMAPCAZQALWBYAE1AAOQB5MlBfMUQAXzboK2xg0PC7B1BnGnpJQ01tRNBkq1TMDNhsxzTkWAAxRwAPTFqvACY-MrAqmuh65tb2oS6gA

In the case of a union type for tuple (which, in the example above, I use to simulate a Maybe type), flow can't refine the type in the else branch even though we're guaranteed at that point that the shape of y is [null, number], yet it errors because it thinks it could be [Error, number].

Here's a very slight modification that causes the other branch to lose refinement:

https://flow.org/try/#0PTAEAEDMBsHsHcBQAXAngBwKagLIENUAjTAHgBUA+UAXlAG0A7AV2mgBpQyBdUAH3oCiAJyGwhHZqy6JomZKFQAuXAWIlmAW2JCqtRi3agAjNMQBjWAwDO8uphEcAlj1qpEjyKAAUjmtVqS0ACUoADeiKCgIPT24qDONIIiYhIG0pEW1rCyAHRwAOZesTkamFZWePmYIdEAygAWsCwAJqAAcgDyZKCxYogAvj3QVtjhkdF2DvEu9IGpWvbpoJlW2Zh5sIWOOciwAGKOAB6YzV4ATEE1YA1N0K2d3b1CA0A

[bradennapier]

#1414 #6429 #6464 #6565 #6666 #4296 #6677 #6408

I didnt look at those but I have run into it countless times so I know there are lots of unanswered mentions of it dating back many months. Just wanted to connect them for referencing