.tsp: drop the single-null debugger stop

Avoid interactive startup pauses when `get_null_segs()` sees one
zero-time row. Remove the explicit breakpoint and its now-unused
`RuntimeError` handler, preserving the legacy `None` result.

LOCAL UX CLEANUP -- NOT the downstream null-grouping fix.
The related source below removes this branch as part of a much
larger grouping/deduplication rewrite. None of that algorithm,
its index validation or its oracle coverage is backported here.

Downstream replay,
Prefer the complete downstream implementation. Do not preserve
this legacy single-null early return against the later rewrite.
The small inline comment records that temporary discrepancy.

Related-downstream: 976db6c33c
Prompt-IO: ai/prompt-io/opencode/20261022T212248Z_dc618229_ib_recovery_prompt_io.md

(this patch was generated in some part by `opencode` using
`gpt-6-astra` (`openai`))

piker/tsp/_anal.py

@@ -276,15 +276,8 @@ def get_null_segs(
         absi_zdiff: np.ndarray = np.diff(absi_zeros)
 
         if zero_t.size < 2:
-            try:
+            # Keep the legacy single-null result without a REPL stop.
-                breakpoint()
+            # Full null grouping is handled downstream in 976db6c3.
-            except RuntimeError:
-                # XXX, if greenback not active from
-                # piker store ldshm cmd..
-                log.exception(
-                    "Can't debug single-sample null!\n"
-                )
-
             return None
 
         # scan for all frame-indices where the