Correctly handle zero explicit_port

Issue: aio-libs#1408

CHANGES/1413.bugfix.rst

@@ -0,0 +1,2 @@
+Fixed a bug causing ``URL.port`` to return the default port when the given port was zero
+-- by :user:`gmacon`.

tests/test_url.py

@@ -380,6 +380,12 @@ def test_port_for_unknown_scheme():
     assert url.explicit_port is None
 
 
+def test_explicit_zero_port():
+    url = URL("http://example.com:0")
+    assert url.explicit_port == 0
+    assert url.port == 0
+
+
 def test_explicit_port_for_explicit_port():
     url = URL("http://example.com:8888")
     assert 8888 == url.explicit_port

yarl/_url.py

@@ -784,7 +784,10 @@ def port(self) -> Union[int, None]:
         scheme without default port substitution.
 
         """
-        return self.explicit_port or self._default_port
+        port = self.explicit_port
+        if port is None:
+            port = self._default_port
+        return port
 
     @cached_property
     def explicit_port(self) -> Union[int, None]: