TSSource: use None instead of floats for default time values

t0 = 0 corresponds to an actual time, which is conceptually different
from an unspecified time, and some source elements may need to behave
differently for unspecified times.

For consistency we make the same default change for "end" as well.

src/sgnts/base/__init__.py

@@ -425,19 +425,20 @@ class TSSource(SourceElement):
 
     """
 
-    t0: float = 0
-    end: float = float("+inf")
+    t0: float | None = None
+    end: float | None = None
 
     def __post_init__(self):
         super().__post_init__()
+        t0 = self.t0 or 0
         # FIXME should we be more careful about this?
         # FIXME should this not be different by pad?
         self.offset = {
-            p: Offset.fromsec(self.t0 - Offset.offset_ref_t0 / Time.SECONDS)
+            p: Offset.fromsec(t0 - Offset.offset_ref_t0 / Time.SECONDS)
             for p in self.source_pads
         }
         # FIXME should this be different by pad?
-        if not isinf(self.end):
+        if self.end is not None and not isinf(self.end):
             self.end_offset = Offset.fromsec(
                 self.end - Offset.offset_ref_t0 / Time.SECONDS
             )