Backend/Ether: refactor null checks

Make all null checks use isNull function as it's the most
reliable way to test if value is null.

src/GWallet.Backend/Ether/EtherServer.fs

@@ -179,7 +179,7 @@ module Server =
                 raise <| ServerUnavailableException(exMsg, httpReqEx)
 
             // TODO: report this one as a warning to sentry?
-            if httpReqEx.InnerException <> null &&
+            if not (isNull httpReqEx.InnerException) &&
                httpReqEx.InnerException :? System.Runtime.InteropServices.COMException then
                 // got this once, with the exception message
                 // "the text associated with this error code could not be found.
@@ -302,7 +302,7 @@ module Server =
 
                 // FIXME: report this UWP bug (see commit message for full stacktrace)
                 if Xamarin.Essentials.DeviceInfo.Platform.Equals Xamarin.Essentials.DevicePlatform.UWP &&
-                   httpReqEx.InnerException <> null && httpReqEx.InnerException.GetType() = typeof<Exception> then
+                   not (isNull httpReqEx.InnerException) && httpReqEx.InnerException.GetType() = typeof<Exception> then
                     raise <| ServerCannotBeResolvedException(httpReqEx.InnerException.Message, ex)
 *)
 
@@ -567,7 +567,7 @@ module Server =
                                 let task = web3.Eth.GetBalance.SendRequestAsync (address, null, cancelToken)
                                 return! Async.AwaitTask task
                             }
-                    if Object.ReferenceEquals(balance, null) then
+                    if isNull balance then
                         raise <| 
                             AbnormalNullValueInJsonResponseException 
                                 AbnormalNullValueInJsonResponseException.BalanceJobErrorMessage