Fix Pinboard RSS parsing valid links as `None`

`item.find(p)` returns either an `ElementTree.Element` or `None`.  The
[lambda on line 24][lambda] coerces the return value to a bool, which is
`False` if the `<link>` element has no children (see
[`ElementTree.py` line 207][etbooldef]), so the lambda returns `None`.

Further, returning a `Link` with `url=None` violates
[an assertion in `index/schema.py`][assertion], which crashes
the `archivebox add` command.

[lambda]: https://github.com/ArchiveBox/ArchiveBox/blob/3d54b1321bf8c56627aaa50efcc809cd99caee52/archivebox/parsers/pinboard_rss.py#L24
[etbooldef]: https://github.com/python/cpython/blob/3d8993a744813c5144851da5347d7b4b1885f234/Lib/xml/etree/ElementTree.py#L207
[assertion]: https://github.com/ArchiveBox/ArchiveBox/blob/3d54b1321bf8c56627aaa50efcc809cd99caee52/archivebox/index/schema.py#L165

archivebox/parsers/pinboard_rss.py

@@ -21,13 +21,18 @@ def parse_pinboard_rss_export(rss_file: IO[str], **_kwargs) -> Iterable[Link]:
     root = ElementTree.parse(rss_file).getroot()
     items = root.findall("{http://purl.org/rss/1.0/}item")
     for item in items:
-        find = lambda p: item.find(p).text.strip() if item.find(p) else None    # type: ignore
+        find = lambda p: item.find(p).text.strip() if item.find(p) is not None else None    # type: ignore
 
         url = find("{http://purl.org/rss/1.0/}link")
         tags = find("{http://purl.org/dc/elements/1.1/}subject")
         title = find("{http://purl.org/rss/1.0/}title")
         ts_str = find("{http://purl.org/dc/elements/1.1/}date")
         
+        if url is None:
+            # Yielding a Link with no URL will
+            # crash on a URL validation assertion
+            continue
+
         # Pinboard includes a colon in its date stamp timezone offsets, which
         # Python can't parse. Remove it:
         if ts_str and ts_str[-3:-2] == ":":