Avoid linking rows with NULL primary keys

[sb123sb123]

Summary

• Render rows with NULL primary-key values as plain text instead of linking them to an invalid row URL.
• Omit inline Edit and Delete actions for rows whose NULL keys cannot reliably identify a row.
• Preserve links and actions for valid keys, including a literal text key of None.

Fixes #2145

Validation

• uv run --python 3.13 --group dev pytest -q tests/test_table_html.py: 109 passed on macOS.
• ruff check tests/test_table_html.py datasette/views/table.py: passed.
• black --check --target-version py310 tests/test_table_html.py datasette/views/table.py: passed.
• Read the Docs check: passed.

The same full test module hit temporary-directory cleanup errors when run on the separate Windows G-drive environment (97 passed, 12 failed, 1 error); the isolated failure was also reproducible in the unchanged test_table_cell_truncation test. The complete module passes on macOS.