Add response valid error to check getalby connection

[karliatto]

GetAlby has changed their default LNDHub string to point to a different URL and it has some differences in the responses.

Previously the lndhub string provided by GetAbly was like:

"lndhub://:@https://ln.getalby.com"

Now they are like:
"lndhub://:@https://getalby.com/lndhub/"

The new one when not finding a route to a node it responds:

{"payment_hash":{"type":"Buffer","data":[]},"payment_request":"<bolt11-invoice>","pay_req":""<bolt11-invoice>","num_satoshis":100,"description":null,"description_hash":"","payment_error":"","payment_preimage":{"type":"Buffer","data":[]},"payment_route":null}

Instead of the previous one (that is still alive so it is valid):

{"code":10,"error":true,"message":"Payment failed. Does the receiver have enough inbound capacity? (no_route)"}

So adding '"payment_route":null',to the checkMethodErrorMessages in GetAlby fixes the issue.