Keep an empty-string keyset pagination cursor, closes #2967

path_from_row_pks() returns "" for a row whose only primary key value is
the empty string, and "" is a legitimate _next= cursor. Both the site
that emits the token and the site that reads it back tested it for
truthiness, so the token was discarded.

Emitting:

    "next": next_value and str(next_value) or None

turns "" into None, while next_url is still built a few lines earlier
and stays non-null. docs/json_api.rst:99 says both are null on the final
page, so the response contradicted its own documentation. Reading back,
"if _next:" then ignored the cursor and the query returned page 1
again, so a client following next_url looped on the first row forever.

    CREATE TABLE t (id TEXT PRIMARY KEY);
    INSERT INTO t (id) VALUES ('a'), (''), ('c'), ('d'), 'e');

    /d/t.json?_size=1   rows ['']   next=None   next_url=...&_next=
    following it       rows ['']   next=None   next_url=...&_next=

None is the no-cursor sentinel: it is initialised to None at
table.py:2508 and nothing assigns "" to it deliberately. The same
function already asks this correctly at table.py:2536
("if prefix is None: prefix = '\'").

A bare ?_next= with no value now means "rows after the empty primary
key" rather than being ignored, which is the only coherent reading of
an empty cursor. _next=a and _next=zzz are unchanged.

No docs change: json_api.rst already describes the fixed behaviour.

Test: tests/test_table_api.py::test_paginate_table_with_empty_string_primary_key
walks the pages and fails on cec5e6b2 with

    assert '_next=None' in '...?_size=1&_next='

pytest -n auto -m 'not serial' 2782 passed, pytest -m serial 40 passed.
black, ruff, cog --check and codespell clean.

datasette/views/table.py

@@ -2140,10 +2140,10 @@ async def table_view_data(
     count_sql = f"select count(*) {from_sql}"
 
     # Handle pagination driven by ?_next=
-    _next = _next or request.args.get("_next")
+    _next = _next if _next is not None else request.args.get("_next")
 
     offset = ""
-    if _next:
+    if _next is not None:
         sort_value = None
         if is_view:
             # _next is an offset
@@ -2397,7 +2397,7 @@ async def table_view_data(
 
     data = {
         "ok": True,
-        "next": next_value and str(next_value) or None,
+        "next": None if next_value is None else str(next_value),
         "next_url": next_url,
     }
     data.update(

tests/test_table_api.py

@@ -366,6 +366,49 @@ async def test_page_size_zero(ds_client):
     assert None is response.json()["next_url"]
 
 
+def test_paginate_table_with_empty_string_primary_key():
+    # path_from_row_pks() returns "" for a row whose only primary key
+    # value is the empty string, and "" is a legitimate _next= cursor.
+    # The "next" key and the _next= handling both used a truthiness
+    # check, so the token was dropped and pagination never advanced.
+    with make_app_client(
+        memory=True,
+        extra_databases={
+            "empty_pk": (
+                "CREATE TABLE t (id TEXT PRIMARY KEY);"
+                "INSERT INTO t (id) VALUES ('a'), (''), ('c'), ('d'), ('e');"
+            )
+        },
+    ) as client:
+        fetched = []
+        path = "/empty_pk/t.json?_size=1"
+        page = 0
+        while path:
+            page += 1
+            response = client.get(path)
+            assert response.status == 200
+            data = response.json
+            fetched.extend(row["id"] for row in data["rows"])
+            if data["next_url"]:
+                # next_url and next agree, per docs/json_api.rst
+                assert (
+                    urllib.parse.urlencode({"_next": data["next"]}) in data["next_url"]
+                )
+            path = data["next_url"]
+            if path:
+                path = path.replace("http://localhost", "")
+            assert page < 30, "Possible infinite loop detected"
+
+        # Every row exactly once
+        assert 5 == len(fetched)
+        assert ["", "a", "c", "d", "e"] == sorted(fetched)
+        assert 5 == page
+        # And the final page has both keys null, as documented
+        last = client.get("/empty_pk/t.json?_size=100").json
+        assert None is last["next"]
+        assert None is last["next_url"]
+
+
 @pytest.mark.asyncio
 async def test_paginate_compound_keys(ds_client):
     fetched = []