add NullRegularizer

[MaxenceGollier]

closes #155.

[Copilot]

Pull request overview

Adds a NullRegularizer (identically-zero regularizer) so smooth problems can be run through solvers that expect a regularizer/prox interface.

Changes:

• Introduces NullRegularizer implementation with prox!, shifted, and shift!.
• Wires the new operator into the module via include("null.jl").
• Adds tests covering evaluation and prox! behavior for Float64/Float32, including shifted/shift! calls.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/null.jl	Implements the new null regularizer and its prox/shift-related methods.
src/ShiftedProximalOperators.jl	Includes the new null.jl file in the module build.
test/runtests.jl	Adds a new testset validating NullRegularizer behavior.

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[codecov]

Codecov Report

❌ Patch coverage is 76.66667% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.35%. Comparing base (753a97f) to head (6b45e45).
⚠️ Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
src/null.jl	66.66%	8 Missing ⚠️
src/shiftedNullBox.jl	85.71%	5 Missing ⚠️
src/ShiftedProximalOperators.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   73.71%   77.35%   +3.63%     
==========================================
  Files          22       26       +4     
  Lines         898     1029     +131     
==========================================
+ Hits          662      796     +134     
+ Misses        236      233       -3     

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