Bah, fix nursery indents for maybe tsdb backloading

Can't ref `dt_eps` and `tsdb_entry` if they don't exist.. like for 1s
sampling from `binance` (which dne). So make sure to add better logic
guard and only open the finaly backload nursery if we actually need to
fill the gap between latest history and where tsdb history ends.

TO CHERRY #486

piker/tsp/__init__.py

@@ -891,6 +891,12 @@ async def tsdb_backfill(
                 config,
             )
 
+            tsdb_entry: tuple = await load_tsdb_hist(
+                storage,
+                mkt,
+                timeframe,
+            )
+
         # tell parent task to continue
         # TODO: really we'd want this the other way with the
         # tsdb load happening asap and the since the latest
@@ -898,28 +904,25 @@ async def tsdb_backfill(
         # latency?
         task_status.started()
 
-            tsdb_entry: tuple = await load_tsdb_hist(
-                storage,
-                mkt,
-                timeframe,
-            )
-
         # NOTE: iabs to start backfilling from, reverse chronological,
         # ONLY AFTER the first history frame has been pushed to
         # mem!
         backfill_gap_from_shm_index: int = shm._first.value + 1
 
+        # Prepend any tsdb history to the shm buffer which should
+        # now be full of the most recent history pulled from the
+        # backend's last frame.
+        if (
+            dt_eps
+            and tsdb_entry
+        ):
+            # unpack both the latest (gap) backfilled frame dts
             (
                 mr_start_dt,
                 mr_end_dt,
             ) = dt_eps
 
-        async with trio.open_nursery() as tn:
+            # AND the tsdb history from (offline) storage)
-
-            # Prepend any tsdb history to the shm buffer which should
-            # now be full of the most recent history pulled from the
-            # backend's last frame.
-            if tsdb_entry:
             (
                 tsdb_history,
                 first_tsdb_dt,
@@ -929,6 +932,8 @@ async def tsdb_backfill(
             # if there is a gap to backfill from the first
             # history frame until the last datum loaded from the tsdb
             # continue that now in the background
+            async with trio.open_nursery() as tn:
+
                 bf_done = await tn.start(
                     partial(
                         start_backfill,
@@ -941,8 +946,10 @@ async def tsdb_backfill(
 
                         backfill_from_shm_index=backfill_gap_from_shm_index,
                         backfill_from_dt=mr_start_dt,
+
                         sampler_stream=sampler_stream,
                         backfill_until_dt=last_tsdb_dt,
+
                         storage=storage,
                         write_tsdb=True,
                     )
@@ -1032,10 +1039,10 @@ async def tsdb_backfill(
                     mkt=mkt,
                 ))
 
+            # 2nd nursery END
 
             # TODO: who would want to?
             await nulls_detected.wait()
-
             await bf_done.wait()
         # TODO: maybe start history anal and load missing "history
         # gaps" via backend..