labhub.py: Add error message when nick is empty

All of the backends do not have `nick` attribute.
Hence an error will be shown whenever nick is empty.

Closes #632

plugins/labhub.py

@@ -92,6 +92,9 @@ def team_mapping(self):
     @cmdfilter
     def members_only(self, msg, cmd, args, dry_run):
         user = msg.frm.nick
+        if not user:
+            self.send(msg.frm, 'ERROR: The command cannot be operated without using `nick`')
+            return None, None, None
         commands = constants.PRIVATE_CMDS
 
         if cmd in commands:
@@ -122,6 +125,9 @@ def invite_cmd(self, msg, match):
         """
         invitee = match.group(1)
         inviter = msg.frm.nick
+        if not inviter:
+            self.send(msg.frm, 'ERROR: The command cannot be operated without using `nick`')
+            return
 
         team = 'newcomers' if match.group(2) is None else match.group(2)
         team = team.lower()
@@ -180,7 +186,9 @@ def callback_message(self, msg):
         """Invite the user whose message includes the holy 'hello world'"""
         if re.search(r'hello\s*,?\s*world', msg.body, flags=re.IGNORECASE):
             user = msg.frm.nick
-            if (not self.is_team_member(user, 'newcomers')
+            if not user:
+                self.send(msg.frm, 'ERROR: The command cannot be operated without using `nick`')
+            elif (not self.is_team_member(user, 'newcomers')
                     and user not in self.hello_world_users):
                 response = tenv().get_template(
                     'labhub/hello-world.jinja2.md'
@@ -196,6 +204,9 @@ def callback_message(self, msg):
     def create_issue_cmd(self, msg, match):
         """Create issues on GitHub and GitLab repositories."""  # Ignore QuotesBear, LineLengthBear, PyCodeStyleBear
         user = msg.frm.nick
+        if not user:
+            self.send(msg.frm, 'ERROR: The command cannot be operated without using `nick`')
+            return
         repo_name = match.group(1)
         iss_title = match.group(2)
         iss_description = match.group(3) if match.group(3) is not None else ''
@@ -234,6 +245,9 @@ def unassign_cmd(self, msg, match):
         issue_number = match.group(4)
 
         user = msg.frm.nick
+        if not user:
+            self.send(msg.frm, 'ERROR: The command cannot be operated without using `nick`')
+            return
 
         try:
             assert org == self.GH_ORG_NAME or org == self.GL_ORG_NAME
@@ -317,6 +331,9 @@ def assign_cmd(self, msg, match):
         iss_number = match.group(4)
 
         user = msg.frm.nick
+        if not user:
+            self.send(msg.frm, 'ERROR: The command cannot be operated without using `nick`')
+            return
 
         try:
             assert org == self.GH_ORG_NAME or org == self.GL_ORG_NAME