Handle valid null frames and 0-bar cases in backfill

Add guards for empty-array and zero-bar-diff cases in the TSP backfill
loops to avoid crashes and allow graceful loop termination.

In `maybe_fill_null_segments()`,
- add `array.size == 0` guard in `maybe_fill_null_segments()` to detect
  valid (venue closure) gaps from the backend; add a warning + bp
  + break for this case.
- add TODO that we should likely be filling nulls with the close price
  for the gap's duration.

In `start_backfill()`,
- expand the "0 bars after diff" warning msg with
  `backfill_until_dt` and `end_dt_param` context.
- mask the  `await tractor.pause()` and add a `break` to avoid blocking
  the backfill loop.

(this commit msg was generated in some part by
[`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/tsp/_history.py

@@ -251,10 +251,20 @@ async def maybe_fill_null_segments(
             end_dt=end_dt,
         )
 
+        if array.size == 0:
+            log.warning(
+                f'Valid gap from backend ??\n'
+                f'{end_dt} -> {start_dt}\n'
+            )
+            # ?TODO? do we want to remove the nulls and push
+            # the close price here for the gap duration?
+            await tractor.pause()
+            break
+
         if (
-            frame_start_dt := (
-                from_timestamp(array['time'][0])
-            ) < backfill_until_dt
+            frame_start_dt := (from_timestamp(array['time'][0]))
+            <
+            backfill_until_dt
         ):
             log.error(
                 f'Invalid frame_start !?\n'
@@ -616,10 +626,17 @@ async def start_backfill(
 
             else:
                 log.warning(
-                    '0 BARS TO PUSH after diff!?\n'
+                    f'0 BARS TO PUSH after diff!?\n'
                     f'{next_start_dt} -> {last_start_dt}'
+                    f'\n'
+                    f'This might mean we rxed a gap frame which starts BEFORE,\n'
+                    f'backfill_until_dt: {backfill_until_dt}\n'
+                    f'end_dt_param: {end_dt_param}\n'
+
                 )
-                await tractor.pause()
+                # XXX, to debug it and be sure.
+                # await tractor.pause()
+                break
 
             # Check if we're about to exceed buffer capacity BEFORE
             # attempting the push