fix chain config parser regression

fixes #1180

nimbus/chain_config.nim

@@ -245,7 +245,12 @@ template to(a: string, b: type UInt256): UInt256 =
   UInt256.fromHex(a)
 
 proc loadNetworkParams*(cc: CustomChain, cg: var NetworkParams): bool =
+  if cc.genesis.isNil:
+    cg.genesis = Genesis()
+    warn "Loaded custom network configuration contains no 'genesis' data"
+  else:
     cg.genesis = cc.genesis
+
   cg.config                = ChainConfig()
   cg.config.chainId        = cc.config.chainId
   cg.config.daoForkSupport = cc.config.daoForkSupport

tests/customgenesis/nogenesis.json

@@ -0,0 +1,21 @@
+{
+  "config": {
+    "chainId": 7,
+    "homesteadBlock": 0,
+    "eip150Block": 0,
+    "eip150Hash": "0x5de1ee4135274003348e80b788e5afa4b18b18d320a5622218d5c493fedf5689",
+    "eip155Block": 0,
+    "eip158Block": 0,
+    "byzantiumBlock": 0,
+    "constantinopleBlock": 0,
+    "petersburgBlock": 0,
+    "istanbulBlock": 0,
+    "muirGlacierBlock": 0,
+    "berlinBlock": 0,    
+    "londonBlock": 0,
+    "clique": {
+      "epoch": 3000,
+      "period": 1
+    }
+  }
+}

tests/test_configuration.nim

@@ -1,6 +1,6 @@
 import
   std/[os],
-  pkg/[unittest2, confutils],
+  pkg/[unittest2],
   eth/[p2p, common, keys],
   stew/byteutils,
   ../nimbus/[config, chain_config, context],
@@ -12,7 +12,10 @@ proc `==`(a, b: ChainId): bool =
 proc configurationMain*() =
   suite "configuration test suite":
     const
-      genesisFile = "tests" / "customgenesis" / "calaveras.json"
+      jsonDir = "tests" / "customgenesis"
+      genesisFile = jsonDir / "calaveras.json"
+      noGenesis = jsonDir / "nogenesis.json"
+      noConfig = jsonDir / "noconfig.json"
       bootNode = "enode://a24ac7c5484ef4ed0c5eb2d36620ba4e4aa13b8c84684e1b4aab0cebea2ae45cb4d375b77eab56516d34bfbd3c1a833fc51296ff084b770b94fb9028c4d25ccf@52.169.42.101:30303"
 
     test "data-dir and key-store":
@@ -49,6 +52,14 @@ proc configurationMain*() =
       check bb.cmd == NimbusCmd.`import`
       check bb.blocksFile.string == genesisFile
 
+    test "custom-network loading config file with no genesis data":
+      let conf = makeConfig(@["--custom-network:" & noGenesis])
+      check conf.networkParams.genesis.isNil == false
+
+    test "custom-network loading config file with no 'config'":
+      let conf = makeConfig(@["--custom-network:" & noConfig])
+      check conf.networkParams.config.isNil == false
+
     test "network-id":
       let aa = makeTestConfig()
       check aa.networkId == MainNet