Fix some tab-complete regressions

proxy/src/main/java/com/velocitypowered/proxy/command/VelocityCommandManager.java

@@ -167,7 +167,14 @@ public class VelocityCommandManager implements CommandManager {
       if (line.isEmpty()) {
         return new String[0];
       }
-      return line.trim().split(" ", -1);
+
+      String[] trimmed = line.trim().split(" ", -1);
+      if (line.endsWith(" ") && !line.trim().isEmpty()) {
+        // To work around a 1.13+ specific bug we have to inject a space at the end of the arguments
+        trimmed = Arrays.copyOf(trimmed, trimmed.length + 1);
+        trimmed[trimmed.length - 1] = "";
+      }
+      return trimmed;
     }
 
     @Override

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -365,12 +365,12 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
   private boolean handleCommandTabComplete(TabCompleteRequest packet) {
     // In 1.13+, we need to do additional work for the richer suggestions available.
     String command = packet.getCommand().substring(1);
-    int spacePos = command.indexOf(' ');
+    int commandEndPosition = command.indexOf(' ');
-    if (spacePos == -1) {
+    if (commandEndPosition == -1) {
-      spacePos = command.length();
+      commandEndPosition = command.length();
     }
 
-    String commandLabel = command.substring(0, spacePos);
+    String commandLabel = command.substring(0, commandEndPosition);
     if (!server.getCommandManager().hasCommand(commandLabel)) {
       if (player.getProtocolVersion().compareTo(MINECRAFT_1_13) < 0) {
         // Outstanding tab completes are recorded for use with 1.12 clients and below to provide