fix(2-G.5): preview_for_settings contract in adapter docstring + distinguish [] from None

Fixup 1 — Contract section appended to SourceAdapter.preview_for_settings's
docstring. Override authors read adapter.py, not routes.py, so the contract
(pure function of settings; open your own short-lived aiohttp session; None
vs [] semantics) belongs on the base method, not on the GUI stub class.

Fixup 2 — _adapter_preview.html distinguishes [] from None. Previously the
elif test was truthiness (`elif preview_rows`) which collapsed both into
"render nothing". Now uses `elif preview_rows is not none` and special-cases
the empty-list case inside: legend "Preview (0 rows)" with no table; None
still renders nothing at all. Lets adapters signal "query ran, matched zero"
distinctly from "preview not meaningful".

Tests +1:
- test_partial_renders_empty_list — [] yields "Preview (0 rows)" legend,
  no table, no headers. Distinct from the existing None case.

Acceptance:
- 27/27 targeted (preview_hook +1 new, nwis, stream_registry).
- 458/458 full suite.
- (b) framework GUI dir still has zero adapter-name branches.

tests/test_preview_hook.py

@@ -106,3 +106,15 @@ def test_partial_renders_nothing_when_both_none():
     assert "Preview Unavailable" not in out
     # Either empty or only whitespace/newlines from the template.
     assert "Preview (" not in out
+
+
+def test_partial_renders_empty_list():
+    """Empty list -> legend with (0 rows), no table.
+
+    Distinct from None (which renders nothing at all). Lets adapters signal
+    'query ran, matched zero rows' separately from 'preview not meaningful'.
+    """
+    out = _render_partial(preview_rows=[], preview_error=None)
+    assert "Preview (0 rows)" in out
+    assert "<table" not in out
+    assert "<th>" not in out