reviewstats: don't write 'None' if duration is unset

Keep the current reviewstats format: when review duration is unset/unknown,
the line contains only two values separated by one tab and then EOL.

Another way to test/reproduce the fixed issue:
1. add 'lllll : !stats' to tests/datas/irc.txt
2. run 'pytest tests/test_review_stats.py'

review/stats.py

@@ -9,6 +9,29 @@ class ReviewData:
     user_count: int
     duration: int
 
+    def __str__(self):
+        return (
+            f"{self.date.strftime('%Y%m%d-%Hh%M')}\t{self.user_count}"
+            f"{f'\t{self.duration}' if self.duration is not None else ''}"
+            "\n"
+        )
+
+    @staticmethod
+    def load(line: str):
+        datas = re.split(r"\s+", line.strip())
+        if len(datas) == 2:
+            return ReviewData(
+                datetime.strptime(datas[0], "%Y%m%d-%Hh%M"),
+                int(datas[1]),
+                None,
+            )
+
+        return ReviewData(
+            datetime.strptime(datas[0], "%Y%m%d-%Hh%M"),
+            int(datas[1]),
+            int(datas[2]),
+        )
+
 
 class Board:
     def __init__(self, datas={}):
@@ -148,23 +171,7 @@ class ReviewStats:
                 lines = file_handle.read().splitlines()
                 for line in lines:
                     if line.strip() != "":
-                        datas = re.split(r"\s+", line)
+                        self.datas.append(ReviewData.load(line))
-                        if len(datas) == 2:
-                            self.datas.append(
-                                ReviewData(
-                                    datetime.strptime(datas[0], "%Y%m%d-%Hh%M"),
-                                    int(datas[1]),
-                                    None,
-                                )
-                            )
-                        else:
-                            self.datas.append(
-                                ReviewData(
-                                    datetime.strptime(datas[0], "%Y%m%d-%Hh%M"),
-                                    int(datas[1]),
-                                    int(datas[2]) if datas[2]!="" else None,
-                                )
-                            )
         except FileNotFoundError:
             # no file, no stats
             pass
@@ -174,7 +181,4 @@ class ReviewStats:
     def save(self):
         with open(self.filepath, "w") as file_handle:
             for data in self.datas:
-                file_handle.write(
+                file_handle.write(str(data))
-                    f"{data.date.strftime('%Y%m%d-%Hh%M')}\t"
-                    f"{data.user_count}\t{data.duration}\n"
-                )

tests/test_review_data.py

@@ -0,0 +1,8 @@
+from review.stats import ReviewData
+
+
+def test_reviewdata():
+    for filename in ("reviewstats_1.csv", "reviewstats_2.csv"):
+        with open(f"tests/datas/{filename}", "r") as review:
+            for line in review:
+                assert line == str(ReviewData.load(line))