The encoded JSON string is going to start with "ey", unless there's whitespace in the first couple characters.

Also, it seem like the really important point is kind of glossed over. Base64 is not a kind of encryption, it's an encoding that anybody can easily decode. Using it to hide secrets in a GitHub repo is a really really dumb thing to do.

There is actual encryption here. The base64 JSON only encodes the salt and parameters of the key derivation function used to encrypt the data.