Fixed an issue where supplying an empty host to the Client TLS handler would result in an SNI error

Motivation:

Testing on Linux seemed to reveal an `NIOSSLExtraError.invalidSNIHostname` error that was thrown when using `https+unix` URLs, which don't have their hostname filled in. The same NIO failure does not occur on macOS, as it seems that Network.framework does not mind an empty hostname.

Modifications:

Added an extra check to determine if the host is empty before passing it to a `NIOSSLClientHandler`.

Result:

testHTTPSPlusUNIX() no longer fails on Linux.

Author: dimitribouniol

Diff for: Sources/AsyncHTTPClient/HTTPClient.swift

@@ -665,7 +665,7 @@ extension ChannelPipeline {
                 let tlsConfiguration = tlsConfiguration ?? TLSConfiguration.forClient()
                 let context = try NIOSSLContext(configuration: tlsConfiguration)
                 handlers = [
-                    try NIOSSLClientHandler(context: context, serverHostname: key.host.isIPAddress ? nil : key.host),
+                    try NIOSSLClientHandler(context: context, serverHostname: (key.host.isIPAddress || key.host.isEmpty) ? nil : key.host),
                     TLSEventsHandler(completionPromise: handshakePromise),
                 ]
             } else {

Diff for: Sources/AsyncHTTPClient/Utils.swift

@@ -64,7 +64,7 @@ extension ClientBootstrap {
         } else {
             let tlsConfiguration = configuration.tlsConfiguration ?? TLSConfiguration.forClient()
             let sslContext = try NIOSSLContext(configuration: tlsConfiguration)
-            let hostname = (!requiresTLS || host.isIPAddress) ? nil : host
+            let hostname = (!requiresTLS || host.isIPAddress || host.isEmpty) ? nil : host
             let tlsProvider = try NIOSSLClientTLSProvider<ClientBootstrap>(context: sslContext, serverHostname: hostname)
             return NIOClientTCPBootstrap(self, tls: tlsProvider)
         }