debug_traceTransaction RPC tracer null throw exception

[FatTigerWang]

Description

An error occurs when debug_traceTransaction is requested with default parameters (only with txhash), but the request succeeds when "tracer" = "", which should not be correct behavior:

// success
{
    "params": [
        "0xf50d56ca5e77504129d685db24144f2b8f8cb44d83906c09441b1094e2828f3d", // any latest txhash
        {
            "tracer": ""
        }
    ]
}

// failed
{
    "params": [
        "0xf50d56ca5e77504129d685db24144f2b8f8cb44d83906c09441b1094e2828f3d" // any latest txhash
    ]
}

Steps to Reproduce

curl --location 'http://127.0.0.1:8545\
--header 'Content-Type: application/json' \
--data '{
    "jsonrpc": "2.0",
    "id": 0,
    "method": "debug_traceTransaction",
    "params": [
        "0x4869212634482b05728a591d976f53b9167b8c15449537a67ff49cb8374da961" // any latest txhash
    ]
}'

Actual behavior

{
    "jsonrpc": "2.0",
    "error": {
        "code": -32603,
        "message": "Internal error",
        "data": "System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.\n ---> System.ArgumentNullException: Value cannot be null. (Parameter 'key')\n   at System.Collections.Generic.Dictionary`2.FindValue(TKey key)\n   at Nethermind.Consensus.Tracing.GethStyleTracer.CreateOptionsTracer(BlockHeader block, GethTraceOptions options)\n   at Nethermind.Consensus.Tracing.GethStyleTracer.Trace(Block block, Hash256 txHash, CancellationToken cancellationToken, GethTraceOptions options)\n   at Nethermind.Consensus.Tracing.GethStyleTracer.Trace(Hash256 txHash, GethTraceOptions traceOptions, CancellationToken cancellationToken)\n   at Nethermind.JsonRpc.Modules.DebugModule.DebugRpcModule.debug_traceTransaction(Hash256 transactionHash, GethTraceOptions options)\n   at InvokeStub_IDebugRpcModule.debug_traceTransaction(Object, Span`1)\n   at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)\n   --- End of inner exception stack trace ---\n   at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)\n   at Nethermind.JsonRpc.JsonRpcService.ExecuteAsync(JsonRpcRequest request, String methodName, ResolvedMethodInfo method, JsonRpcContext context)"
    },
    "id": 0
}

Expected behavior

{
    "jsonrpc": "2.0",
    "result": {
        "gas": "0x9d9c",
        "failed": false,
        "returnValue": "0x0000000000000000000000000000000000000000000000000000000000000001",
        "structLogs": [
            {
                "pc": 0,
                "op": "PUSH1",
                "gas": 23766,
                "gasCost": 3,
                "depth": 1,
                "error": null,
                "stack": [],
                "storage": {}
            },
            ...
       ]
   }
}