More work on CHAT

Always add group 0 info to output of API call GET /chats/active/{address}.
No groupName entry as it's "no group" or "group-less" or "not group related".
Timestamp also might be omitted if no message found.

Fix output of POST /chats/compute so it doesn't include zeroed 64-byte signature.

src/main/java/org/qortal/api/resource/ChatResource.java

@@ -227,10 +227,14 @@ public class ChatResource {
 
 			chatTransaction.computeNonce();
 
+			// Re-check, but ignores signature
 			result = chatTransaction.isValidUnconfirmed();
 			if (result != ValidationResult.OK)
 				throw TransactionsResource.createTransactionInvalidException(request, result);
 
+			// Strip zeroed signature
+			transactionData.setSignature(null);
+
 			byte[] bytes = ChatTransactionTransformer.toBytes(transactionData);
 			return Base58.encode(bytes);
 		} catch (TransformationException e) {

src/main/java/org/qortal/data/chat/ActiveChats.java

@@ -12,13 +12,14 @@ public class ActiveChats {
 	public static class GroupChat {
 		private int groupId;
 		private String groupName;
-		private long timestamp;
+		// Might not be present for groupId 0
+		private Long timestamp;
 
 		protected GroupChat() {
 			/* JAXB */
 		}
 
-		public GroupChat(int groupId, String groupName, long timestamp) {
+		public GroupChat(int groupId, String groupName, Long timestamp) {
 			this.groupId = groupId;
 			this.groupName = groupName;
 			this.timestamp = timestamp;
@@ -32,7 +33,7 @@ public class ActiveChats {
 			return this.groupName;
 		}
 
-		public long getTimestamp() {
+		public Long getTimestamp() {
 			return this.timestamp;
 		}
 	}

src/main/java/org/qortal/repository/hsqldb/HSQLDBChatRepository.java

@@ -145,6 +145,28 @@ public class HSQLDBChatRepository implements ChatRepository {
 			throw new DataException("Unable to fetch active group chats from repository", e);
 		}
 
+		// We need different SQL to handle group-less chat
+		String grouplessSql = "SELECT created_when "
+				+ "FROM ChatTransactions "
+				+ "JOIN Transactions USING (signature) "
+				+ "WHERE tx_group_id = 0 "
+				+ "AND recipient IS NULL "
+				+ "ORDER BY created_when DESC "
+				+ "LIMIT 1";
+
+		try (ResultSet resultSet = this.repository.checkedExecute(grouplessSql)) {
+			Long timestamp = null;
+
+			if (resultSet != null)
+				// We found a recipient-less, group-less CHAT message, so report its timestamp
+				timestamp = resultSet.getLong(1);
+
+			GroupChat groupChat = new GroupChat(0, null, timestamp);
+			groupChats.add(groupChat);
+		} catch (SQLException e) {
+			throw new DataException("Unable to fetch active group chats from repository", e);
+		}
+
 		return groupChats;
 	}