feat(conditions): add :nullAddress as automatically injected context variable

[theref]

Summary

Adds :nullAddress as an automatically injected context variable that resolves to the Ethereum null address (0x0000000000000000000000000000000000000000) on the node side.

Motivation

This is the taco-web companion PR to nucypher/nucypher#3668, which adds the :nullAddress protected context variable to the nucypher node implementation.

Changes

• Add :nullAddress to AUTOMATICALLY_INJECTED_CONTEXT_PARAMS array
• Add :nullAddress to RESERVED_CONTEXT_PARAMS array to prevent manual setting by users

Testing

The existing test suite already handles automatically injected parameters:

• Tests verify that automatically injected parameters cannot be manually set
• Tests verify that reserved parameters are properly protected

Related PRs

• nucypher/nucypher#3668 - Node-side implementation of :nullAddress context variable

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :warning: Please upload report for BASE (signing-epic@552cf6f). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff               @@
##             signing-epic     #738   +/-   ##
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  Coverage                ?   89.74%           
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  Files                   ?       97           
  Lines                   ?     8445           
  Branches                ?      291           
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  Hits                    ?     7579           
  Misses                  ?      863           
  Partials                ?        3           

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