fix(constraints): freeze empty constraint groups without reshape error

[Ketchp]

Note

Code and test in this PR was AI aided.

Problem description:

CSRConstraint.from_mutable reshaped con.vars with an inferred -1 dimension. For an empty constraint group (zero rows and zero terms) the vars array has size 0, and NumPy refuses to infer a (0, -1) reshape, raising "cannot reshape array of size 0 into shape (0,newaxis)".

This is a degenerate edge-case, but can be encountered when slicing or masking is used.

Changes proposed in this Pull Request

Pass the explicit _term count instead of -1; NumPy accepts a (0, 0) reshape and the rest of the method already handles zero-row input.

Minimal reproducible example:

import linopy

m = linopy.Model(freeze_constraints=True) # does not work
# m = linopy.Model(freeze_constraints=False)  # works
x = m.add_variables(coords=[range(3)], dims=["t"], name="x")
m.add_constraints(x.isel(t=range(0)) == 0)  # raises: "ValueError: cannot reshape array of size 0 into shape (0,newaxis)"

Checklist

• AI-generated content is marked (see AGENTS.md).
• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.