System.Text.Json - Move IsReadOnly check inside if-block to avoid cascading if-else

[khellang]

Addresses PR feedback from #40438 (comment).

// @stephentoub

[khellang]

/azp run corefx-ci

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ahsonkhan]

Unrelated test failures in
System.Net.Http.Functional.Tests.SocketsHttpHandlerTest_Http2.Http2_PendingReceive_SendsReset(doRead: False)
https://github.com/dotnet/corefx/issues/40474

System.IO.IOException : Unable to read data from the transport connection: Broken pipe.\n---- System.Net.Sockets.SocketException : Broken pipe

Stack trace
   at System.Net.Security.SslStream.<WriteSingleChunk>g__CompleteAsync|212_1[TWriteAdapter](ValueTask writeTask, Byte[] bufferToReturn) in /_/src/System.Net.Security/src/System/Net/Security/SslStream.Implementation.cs:line 1236
   at System.Net.Security.SslStream.WriteAsyncInternal[TWriteAdapter](TWriteAdapter writeAdapter, ReadOnlyMemory`1 buffer) in /_/src/System.Net.Security/src/System/Net/Security/SslStream.Implementation.cs:line 1486
   at System.Net.Test.Common.Http2LoopbackConnection.WriteFrameAsync(Frame frame) in /_/src/Common/tests/System/Net/Http/Http2LoopbackConnection.cs:line 88
   at System.Net.Http.Functional.Tests.HttpClientHandlerTest_Http2.<>c__DisplayClass62_0.<<Http2_PendingReceive_SendsReset>b__1>d.MoveNext() in /_/src/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.Http2.cs:line 1784
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.Tasks.TaskTimeoutExtensions.WhenAllOrAnyFailed(Task[] tasks) in /_/src/Common/tests/System/Threading/Tasks/TaskTimeoutExtensions.cs:line 83
   at System.Threading.Tasks.TaskTimeoutExtensions.WhenAllOrAnyFailed(Task[] tasks) in /_/src/Common/tests/System/Threading/Tasks/TaskTimeoutExtensions.cs:line 111
   at System.Threading.Tasks.TaskTimeoutExtensions.WhenAllOrAnyFailed(Task[] tasks, Int32 millisecondsTimeout) in /_/src/Common/tests/System/Threading/Tasks/TaskTimeoutExtensions.cs:line 71
   at System.Net.Test.Common.Http2LoopbackServer.CreateClientAndServerAsync(Func`2 clientFunc, Func`2 serverFunc, Int32 timeout) in /_/src/Common/tests/System/Net/Http/Http2LoopbackServer.cs:line 186
   at System.Net.Http.Functional.Tests.HttpClientHandlerTest_Http2.Http2_PendingReceive_SendsReset(Boolean doRead) in /_/src/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.Http2.cs:line 1736
--- End of stack trace from previous location where exception was thrown ---
----- Inner Stack Trace -----

and

System.Net.Sockets.Tests.SocketOptionNameTest.BindDuringTcpWait_Succeeds
https://github.com/dotnet/corefx/issues/40475

System.Net.Sockets.SocketException : Address already in use

Stack trace
   at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, String callerName) in /_/src/System.Net.Sockets/src/System/Net/Sockets/Socket.cs:line 5202
   at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress) in /_/src/System.Net.Sockets/src/System/Net/Sockets/Socket.cs:line 762
   at System.Net.Sockets.Socket.Bind(EndPoint localEP) in /_/src/System.Net.Sockets/src/System/Net/Sockets/Socket.cs:line 703
   at System.Net.Sockets.Tests.SocketOptionNameTest.BindDuringTcpWait_Succeeds() in /_/src/System.Net.Sockets/tests/FunctionalTests/SocketOptionNameTest.cs:line 445

https://dev.azure.com/dnceng/public/_build/results?buildId=316633