core: Populate the core.Exchange map fields even when not connected (#3103)

Returning null here was causing UI errors. It's better to return an
empty map then null.

Author: martonp

client/core/core.go

@@ -479,6 +479,9 @@ func (c *Core) exchangeInfo(dc *dexConnection) *Exchange {
 			AcctID:           acctID,
 			ConnectionStatus: dc.status(),
 			Disabled:         dc.acct.isDisabled(),
+			Markets:          make(map[string]*Market),
+			Assets:           make(map[uint32]*dex.Asset),
+			BondAssets:       make(map[string]*BondAsset),
 		}
 	}