JsonBUtil: Throw exception if CompletableFuture is missing a generic type definition.

Author: Haydon Perrin

http-generator-core/src/main/java/io/avaje/http/generator/core/JsonBUtil.java

@@ -51,7 +51,11 @@ private static void addJsonBodyType(MethodReader methodReader, Consumer<UType> a
 
   public static void writeJsonbType(UType type, Append writer) {
     // Support for CompletableFuture's.
-    if (type.isGeneric() && type.mainType().equals("java.util.concurrent.CompletableFuture")) {
+    if (type.mainType().equals("java.util.concurrent.CompletableFuture")) {
+      if (!type.isGeneric()) {
+        throw new IllegalStateException("CompletableFuture usages must have a generic type defined. e.g. `CompletableFuture<String>` or `CompletableFuture<Void>`.");
+      }
+
       writeJsonbType(type.paramRaw(), writer);
       return;
     }