Fix the FromJSON instance for Payload

[enobayram]

This PR fixes a regression introduced by #52. The rewrite of the FromJSON instance in that PR changed the behavior when the exec field doesn't exist.

[enobayram]

Before #52

ghci> :set -XOverloadedStrings 
ghci> :set -XTypeApplications 
ghci> import Chainweb.Api.Payload
ghci> import Data.Aeson
ghci> eitherDecode @Payload "{}"
Left "Error in $: Payload must be exec or cont"
ghci> eitherDecode @Payload "{ \"exec\": null}"
Left "Error in $: Payload must be exec or cont"
ghci> eitherDecode @Payload "{ \"exec\": {\"code\": \"abc\" } }"
Left "Error in $: key \"data\" not found"
ghci> eitherDecode @Payload "{ \"exec\": {\"code\": \"abc\", \"data\":null } }"
Right (ExecPayload (Exec {_exec_code = "abc", _exec_data = Nothing}))
ghci> eitherDecode @Payload "{ \"cont\": { \"pactId\": \"abc\", \"rollback\": true, \"step\":42, \"data\": [] } }" 
Left "Error in $: key \"proof\" not found"
ghci> eitherDecode @Payload "{ \"cont\": { \"pactId\": \"abc\", \"rollback\": true, \"step\":42, \"data\": [], \"proof\":null } }"
Right (ContPayload (Cont {_cont_pactId = "abc", _cont_rollback = True, _cont_step = 42, _cont_data = Array [], _cont_proof = Nothing}))
ghci> 

After #52

ghci> eitherDecode @Payload "{}"
Left "Error in $: key \"exec\" not found"
ghci> eitherDecode @Payload "{ \"exec\": null}"
Left "Error in $: key \"cont\" not found"
ghci> eitherDecode @Payload "{ \"exec\": {\"code\": \"abc\" } }"
Left "Error in $.exec: key \"data\" not found"
ghci> eitherDecode @Payload "{ \"exec\": {\"code\": \"abc\", \"data\":null } }"
Right (ExecPayload (Exec {_exec_code = "abc", _exec_data = Nothing}))
ghci> eitherDecode @Payload "{ \"cont\": { \"pactId\": \"abc\", \"rollback\": true, \"step\":42, \"data\": [] } }" 
Left "Error in $: key \"exec\" not found"
ghci> eitherDecode @Payload "{ \"cont\": { \"pactId\": \"abc\", \"rollback\": true, \"step\":42, \"data\": [], \"proof\":null } }"
Left "Error in $: key \"exec\" not found"

Fixed with this PR:

ghci> eitherDecode @Payload "{}"
Left "Error in $: Payload must be exec or cont"
ghci> eitherDecode @Payload "{ \"exec\": null}"
Left "Error in $: Payload must be exec or cont"
ghci> eitherDecode @Payload "{ \"exec\": {\"code\": \"abc\" } }"
Left "Error in $.exec: key \"data\" not found"
ghci> eitherDecode @Payload "{ \"exec\": {\"code\": \"abc\", \"data\":null } }"
Right (ExecPayload (Exec {_exec_code = "abc", _exec_data = Nothing}))
ghci> eitherDecode @Payload "{ \"cont\": { \"pactId\": \"abc\", \"rollback\": true, \"step\":42, \"data\": [] } }"
Left "Error in $.cont: key \"proof\" not found"
ghci> eitherDecode @Payload "{ \"cont\": { \"pactId\": \"abc\", \"rollback\": true, \"step\":42, \"data\": [], \"proof\":null } }"
Right (ContPayload (Cont {_cont_pactId = "abc", _cont_rollback = True, _cont_step = 42, _cont_data = Array [], _cont_proof = Nothing}))

[emmanueldenloye]

LGTM