Don't crash when creating test data with 0 contacts

If no contacts exist, at least one will be created

briar-core/src/main/java/org/briarproject/briar/test/TestDataCreatorImpl.java

@@ -116,9 +116,6 @@ public class TestDataCreatorImpl implements TestDataCreator {
 	@Override
 	public void createTestData(int numContacts, int numPrivateMsgs,
 			int numBlogPosts, int numForums, int numForumPosts) {
-		if (numContacts == 0)
-			throw new IllegalArgumentException(
-					"Number of contacts must be >= 1");
 		ioExecutor.execute(() -> {
 			try {
 				createTestDataOnIoExecutor(numContacts, numPrivateMsgs,
@@ -133,7 +130,13 @@ public class TestDataCreatorImpl implements TestDataCreator {
 	private void createTestDataOnIoExecutor(int numContacts, int numPrivateMsgs,
 			int numBlogPosts, int numForums, int numForumPosts)
 			throws DbException {
-		List<Contact> contacts = createContacts(numContacts);
+		List<Contact> contacts;
+		if (numContacts == 0) {
+			contacts = new ArrayList<>(contactManager.getActiveContacts());
+			if (contacts.isEmpty()) contacts = createContacts(1);
+		} else {
+			contacts = createContacts(numContacts);
+		}
 		createPrivateMessages(contacts, numPrivateMsgs);
 		createBlogPosts(contacts, numBlogPosts);
 		List<Forum> forums = createForums(contacts, numForums, numForumPosts);