fix async continuous batching

[aniketmaurya]

What does this PR do?

Libraries like vLLM sometimes return an empty list in the step call. This PR handles that.

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• Did you write any new necessary tests?

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90%. Comparing base (49cedbd) to head (332c89f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #429   +/-   ##
===================================
  Coverage    89%    90%           
===================================
  Files        31     31           
  Lines      2025   2026    +1     
===================================
+ Hits       1806   1817   +11     
+ Misses      219    209   -10     

[ali-alshaar7]

should we reorder this one too

LitServe/src/litserve/loops/continuous_batching_loop.py

Line 131 in 332c89f

self.response_queue_ids[uid] = response_queue_id

?