Fix SslStream.WriteAsync with 0-byte write (#13384)

A previous fix added an early exit when 0-byte writes aren't supported by the underlying SSL implementation.  But in doing so, WriteAsync(..., 0) ends up returning a Task that never completes.

Author: stephentoub

src/System.Net.Security/src/System/Net/SecureProtocols/SslStreamInternal.cs

@@ -398,7 +398,7 @@ private void StartWriting(byte[] buffer, int offset, int count, AsyncProtocolReq
                     {
                         // If it's an empty message and the PAL doesn't support that,
                         // we're done.
-                        return;
+                        break;
                     }
 
                     // Request a write IO slot.

src/System.Net.Security/tests/FunctionalTests/SslStreamStreamToStreamTest.cs

@@ -95,7 +95,7 @@ public void SslStream_StreamToStream_Successive_ClientWrite_Sync_Success()
 
         [OuterLoop] // TODO: Issue #11345
         [Fact]
-        public void SslStream_StreamToStream_Successive_ClientWrite_Sync_WithZeroBytes_Success()
+        public void SslStream_StreamToStream_Successive_ClientWrite_WithZeroBytes_Success()
         {
             byte[] recvBuf = new byte[_sampleMsg.Length];
             VirtualNetwork network = new VirtualNetwork();
@@ -110,13 +110,15 @@ public void SslStream_StreamToStream_Successive_ClientWrite_Sync_WithZeroBytes_S
                 Assert.True(result, "Handshake completed.");
 
                 clientSslStream.Write(Array.Empty<byte>());
+                clientSslStream.WriteAsync(Array.Empty<byte>(), 0, 0).Wait();
                 clientSslStream.Write(_sampleMsg);
 
                 serverSslStream.Read(recvBuf, 0, _sampleMsg.Length);
 
                 Assert.True(VerifyOutput(recvBuf, _sampleMsg), "verify first read data is as expected.");
 
                 clientSslStream.Write(_sampleMsg);
+                clientSslStream.WriteAsync(Array.Empty<byte>(), 0, 0).Wait();
                 clientSslStream.Write(Array.Empty<byte>());
 
                 serverSslStream.Read(recvBuf, 0, _sampleMsg.Length);