Make namespaces a generator by default

90f6fe5 changed

```python
class Store:
    def namespaces(self):
	""" """
	if False:
	    yield None
```

to

```python
class Store:
    def namespaces(self):
	""" """
```

This resulted in `Store.namespaces` no longer being an emtpy generator
by default (see https://stackoverflow.com/q/13243766).

The reason it was removed is because it is unreachable code, however I
did not consider that it would make the function an empty generator.

The reason why `if False: yield None` made it an empty generator is
because the presence of the yield keyword informs CPython that a
function is a generator:
https://www.python.org/dev/peps/pep-0255/#why-a-new-keyword-for-yield-why-not-a-builtin-function-instead

There are several ways of making an empty generator:

- `if False: yield`
- `return` and `yield`
- `yield from ()`
- ... more

Both `if False: yield` and `return`; `yield` results in same bytecode
however, which is that of an empty function, and they are both equally
confusing I would say.

`yield from ()` is somewhat clearer, but the bytecode of the funciton
looks very different from an empty function (see https://stackoverflow.com/a/61496399)

So the best option I see is to just put back what was there and add a
comment and a test to prevent the issue from re-occuring.

rdflib/store.py

@@ -364,6 +364,11 @@ def namespace(self, prefix):
 
     def namespaces(self):
         """ """
+        # This is here so that the function becomes an empty generator.
+        # See https://stackoverflow.com/q/13243766 and
+        # https://www.python.org/dev/peps/pep-0255/#why-a-new-keyword-for-yield-why-not-a-builtin-function-instead
+        if False:
+            yield None  # type: ignore[unreachable]
 
     # Optional Transactional methods
 

test/test_store.py

@@ -0,0 +1,18 @@
+import unittest
+from rdflib import Graph, URIRef
+from tempfile import NamedTemporaryFile, TemporaryDirectory
+from pathlib import Path, PurePath
+from rdflib.store import Store
+
+
+class TestAbstractStore(unittest.TestCase):
+    def test_namespaces(self):
+        """
+        This tests that Store.namespaces is an empty generator.
+        """
+        my_store = Store()
+        self.assertEqual(list(my_store.namespaces()), [])
+
+
+if __name__ == "__main__":
+    unittest.main()