Fix out-of-bounds accesses caught by debug-mode assertions

[cleitonaugusto]

Description

The two tests disabled in debug mode for #2199 abort because of real out-of-bounds accesses, not test issues. I reproduced both on GCC 11 by building with -D_GLIBCXX_ASSERTIONS, which is the check GCC 15 turns on in debug builds.

• Partition, "No gadgets": get_partitioned_paulis_for_largest_first_method takes &colour_vec.front() of an empty vector when there are no terms. Changed to colour_vec.data(), which is valid for an empty vector and never read since there are no vertices to colour.
• LinePlacement, "Empty circuit, two qubit Architecture": for an empty circuit interactions_to_lines returns one empty line, which is dropped and counted as an unused node, so remove_worst_nodes is asked to remove 3 nodes from a 2-node architecture. On the last iteration find_worst_node calls min_degree on an empty graph, and std::min_element returns the end iterator, which is then dereferenced. find_worst_node now returns std::nullopt when there are no nodes left, which remove_worst_nodes already handles.

Both tests are re-enabled in debug mode (the #ifdef NDEBUG guards are removed).

Checked locally:

• With -D_GLIBCXX_ASSERTIONS, the full C++ suite passes. Reverting either fix on its own makes its test abort again.
• Release C++ suite and full pytket suite pass.

Bumped TKET_VERSION to 2.1.103.

Related issues

Fixes #2199.

Checklist

• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation. (no API change)
• I have added tests that prove my fix is effective or that my feature works. (re-enabled the existing tests)
• I have updated the changelog with any user-facing changes.

[cqc-alec]

Thanks!