fix: empty for linear expressions with terms

[coroa]

Changes proposed in this Pull Request

The .empty() method was testing for .shape == (0,) which is incorrect, when there
are any other dimensions. Instead here we rely on not .size which is equivalent to np.prod(.shape).

The first commit adds a failing test. The second commit fixes it.

Checklist

• 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.