Add error message for missing release notes

Closes gh-6

release-plugin/src/main/java/io/spring/gradle/plugin/release/CreateGitHubReleaseTask.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,15 +43,16 @@ public abstract class CreateGitHubReleaseTask extends DefaultTask {
 	@Input
 	public abstract Property<String> getVersion();
 
-	@Input
-	public abstract Property<String> getReleaseNotes();
-
 	@Input
 	public abstract Property<String> getBranch();
 
 	@Input
 	public abstract Property<Boolean> getCreateRelease();
 
+	@Input
+	@Optional
+	public abstract Property<String> getReleaseNotes();
+
 	@Input
 	@Optional
 	public abstract Property<String> getVersionPrefix();
@@ -67,12 +68,21 @@ public void createGitHubRelease() {
 		var version = getVersion().get();
 		var versionPrefix = getVersionPrefix().getOrElse("");
 		var branch = getBranch().get();
-		var body = getReleaseNotes().get();
+
 		var createRelease = getCreateRelease().get();
 		if (createRelease && gitHubAccessToken == null) {
 			throw new MissingPropertyException("Please provide an access token with -PgitHubAccessToken=...");
 		}
 
+		var body = getReleaseNotes().getOrNull();
+		if (body == null) {
+			// @formatter:off
+			throw new MissingPropertyException(("Nothing was generated by the release-notes-generator, " +
+				"perhaps because no issues were available in release milestone %s. " +
+				"Please ensure there is at least one issue in the release.").formatted(version));
+			// @formatter:on
+		}
+
 		System.out.printf("%sCreating GitHub release for %s/%s@%s%s%n", createRelease ? "" : "[DRY RUN] ",
 				repository.owner(), repository.name(), versionPrefix, version);
 		System.out.printf("%nRelease Notes:%n%n----%n%s%n----%n%n", body.trim());