Fix inconsistency in pick_fastest, add deprecation warning for future…

… behavior change (#476)

fastf1/core.py

@@ -2844,18 +2844,32 @@ def pick_fastest(self, only_by_time: bool = False) -> "Lap":
         Returns:
             instance of :class:`Lap`
         """
+        # TODO: Deprecate returning empty lap object when there is no lap
+        # that matches definion
         if only_by_time:
             laps = self  # all laps
         else:
             # select only laps marked as personal fastest
-            laps = self.loc[self['IsPersonalBest'] == True]  # noqa: E712 comparison with True
+            laps = self.loc[self['IsPersonalBest'] == True]  # noqa: E712
 
         if not laps.size:
+            warnings.warn(("None will be returned instead of an empty Lap "
+                           "object when there are no laps that satisfies "
+                           "the definition for fastest lap starting from "
+                           "version 3.3"),
+                          DeprecationWarning)
+            return Lap(index=self.columns, dtype=object).__finalize__(self)
+
+        if laps['LapTime'].isna().all():
+            warnings.warn(("None will be returned instead of an empty Lap "
+                           "object when there is no recorded LapTime for "
+                           "any lap starting from version 3.3"),
+                          DeprecationWarning)
             return Lap(index=self.columns, dtype=object).__finalize__(self)
 
         lap = laps.loc[laps['LapTime'].idxmin()]
         if isinstance(lap, pd.DataFrame):
-            # More laps, same time
+            # Multiple laps, same time
             lap = lap.iloc[0]  # take first clocked
 
         return lap