Allow optional fields in the uncrashable config file

[monitz87]

Firstly, the docs don't specify that networkConfig and networkConfig.entityIdPrefixes must be defined and cannot be empty. Same thing goes for entitySettings.<ENTITY_NAME>.useDefault (though it can be empty, it must always be defined)

Secondly, these fields (IMO) should be optional. If I'm only managing one chain in my subgraph, and I don't want any prefix, a sensible default would be to assign empty string as a prefix for any chain. useDefault could very well default to {}.