Make graph range field optional in JSON schema.
Make graph range field optional in JSON schema.

Currently, newly created dashboards cannot be saved until setting the
graph range at least once.

jonnenauha/promdash,lborguetti/promdash,prometheus/promdash,thooams/promdash,lborguetti/promdash,prometheus/promdash,lborguetti/promdash,thooams/promdash,jonnenauha/promdash,jmptrader/promdash,alonpeer/promdash,juliusv/promdash,jonnenauha/promdash,prometheus/promdash,juliusv/promdash,jmptrader/promdash,jmptrader/promdash,prometheus/promdash,alonpeer/promdash,thooams/promdash,thooams/promdash,alonpeer/promdash,jmptrader/promdash,jonnenauha/promdash,lborguetti/promdash,juliusv/promdash,alonpeer/promdash,juliusv/promdash