Allow generation of connection URI to work when no conn type (#26765)

Previously if get_uri was called it would fail with `NoneType not iterable`, because of the check `if '-' in conn_type`.

airflow/models/connection.py

@@ -206,13 +206,16 @@ def _parse_from_uri(self, uri: str):
 
     def get_uri(self) -> str:
         """Return connection in URI format"""
-        if "_" in self.conn_type:
+        if self.conn_type and "_" in self.conn_type:
             self.log.warning(
                 "Connection schemes (type: %s) shall not contain '_' according to RFC3986.",
                 self.conn_type,
             )
 
-        uri = f"{str(self.conn_type).lower().replace('_', '-')}://"
+        if self.conn_type:
+            uri = f"{self.conn_type.lower().replace('_', '-')}://"
+        else:
+            uri = "//"
 
         authority_block = ""
         if self.login is not None:

tests/always/test_connection.py

@@ -735,3 +735,11 @@ def test_extra_warnings_non_json(self):
     def test_extra_warnings_non_dict_json(self):
         with pytest.warns(DeprecationWarning, match="not parse as a dictionary"):
             Connection(conn_id="test_extra", conn_type="none", extra='"hi"')
+
+    def test_get_uri_no_conn_type(self):
+        # no conn type --> scheme-relative URI
+        assert Connection().get_uri() == "//"
+        # with host, still works
+        assert Connection(host="abc").get_uri() == "//abc"
+        # parsing back as conn still works
+        assert Connection(uri="//abc").host == "abc"