fixed handling empty tags in TXT file

tests/test_message.py

@@ -556,6 +556,15 @@ This is an answer.
 This is a question.
 {Answer.txt_header}
 This is an answer.
+""")
+        self.file_txt_tags_empty = tempfile.NamedTemporaryFile(delete=False, suffix='.txt')
+        self.file_path_txt_tags_empty = pathlib.Path(self.file_txt_tags_empty.name)
+        with open(self.file_path_txt_tags_empty, "w") as fd:
+            fd.write(f"""TAGS:
+{Question.txt_header}
+This is a question.
+{Answer.txt_header}
+This is an answer.
 """)
         self.file_yaml = tempfile.NamedTemporaryFile(delete=False, suffix='.yaml')
         self.file_path_yaml = pathlib.Path(self.file_yaml.name)
@@ -594,6 +603,10 @@ This is an answer.
         tags = Message.tags_from_file(self.file_path_txt_no_tags)
         self.assertSetEqual(tags, set())
 
+    def test_tags_from_file_txt_tags_empty(self) -> None:
+        tags = Message.tags_from_file(self.file_path_txt_tags_empty)
+        self.assertSetEqual(tags, set())
+
     def test_tags_from_file_yaml(self) -> None:
         tags = Message.tags_from_file(self.file_path_yaml)
         self.assertSetEqual(tags, {Tag('tag1'), Tag('tag2'), Tag('ptag3')})

tests/test_tags.py

@@ -44,6 +44,10 @@ class TestTagLine(CmmTestCase):
         tags = tagline.tags()
         self.assertEqual(tags, {Tag('atag1'), Tag('btag2')})
 
+    def test_tags_empty(self) -> None:
+        tagline = TagLine('TAGS:')
+        self.assertSetEqual(tagline.tags(), set())
+
     def test_tags_with_newline(self) -> None:
         tagline = TagLine('TAGS: tag1\n tag2')
         tags = tagline.tags()