Fix provenance of empty unbound inputs

[LanderOtto]

This commit resolves an issue where None tokens of unbound input ports lacked persistent id, leading to invalid entries in the provenance database.

The commit introduces an InputInjectorStep to persist None tokens and adds a new step, BroadcastTransformer, to handle distributing a single token across multiple output ports.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.76%. Comparing base (05aee38) to head (79974a9).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
streamflow/workflow/transformer.py	55.55%	2 Missing and 2 partials ⚠️
streamflow/workflow/step.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #968      +/-   ##
==========================================
+ Coverage   73.75%   73.76%   +0.01%     
==========================================
  Files          91       91              
  Lines       12469    12493      +24     
  Branches     2181     2187       +6     
==========================================
+ Hits         9196     9216      +20     
- Misses       2746     2748       +2     
- Partials      527      529       +2     

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