[GraphQL] Make issued & executed fields nullable

[jamesdphillips]

Expected Behavior

If the value of the field is 0, return null instead of a timestamp.

Current Behavior

value is returned as Jan 1, 1970 ...

Possible Solution

Update:

• https://github.com/sensu/sensu-go/blob/main/backend/apid/graphql/schema/check.graphql#L203-L205
• https://github.com/sensu/sensu-go/blob/main/backend/apid/graphql/schema/check.graphql#L197-L199

[asachs01]

Adding an internal ref here: https://secure.helpscout.net/conversation/1782919033/28310?folderId=1931916.