Support a SortingAnalyzer with no unit in the core extensions

[adityasingh2400]

Fixes #3679

A SortingAnalyzer built from a sorting with no unit crashes on the first extension with ValueError: need at least one array to concatenate, which is the traceback reported in the issue. This comes up in real pipelines when a curation step removes every unit, for example when one shank of a multishank recording yields nothing. @alejoe91 confirmed in the issue that an analyzer with no unit is still a valid analyzer and should be handled.

There are three places where the zero-unit case falls over. In random_spikes_selection, the per-unit loop never runs so np.concatenate gets an empty list. In extract_waveforms_to_single_buffer and in estimate_templates_with_accumulator, the sparse branch computes int(max(np.sum(sparsity_mask, axis=1))) on a mask that has no row, so builtin max raises on the empty iterable. estimate_templates_with_accumulator additionally asserts spikes.size > 0 up front.

The fix returns empty results at each of those points. random_spikes_selection returns an empty int64 array, the two channel-count computations use np.max(..., initial=0), and estimate_templates_with_accumulator returns zero-filled templates when there is no spike rather than allocating a zero-sized shared memory buffer. That last change also covers the case where units exist but all of them have zero spikes, which previously tripped the same assert. With these, random_spikes, noise_levels, waveforms and templates all run on an empty analyzer in both dense and sparse mode, and the analyzer round-trips through the memory, binary_folder and zarr backends.

Tested with test_analyzer_with_no_unit (parametrized over sparse and dense) and test_analyzer_with_only_empty_units in test_sortinganalyzer.py, plus test_random_spikes_selection_no_unit in test_sorting_tools.py covering all four selection methods. Six of the seven new cases fail on main and all pass with the fix. The full test_sortinganalyzer.py, test_sorting_tools.py, test_waveform_tools.py and test_analyzer_extension_core.py suites pass together, 60 passed.