Update gettz tests

Added test for gettz returning local when passed None, empty string or colon.

dateutil/test/test_tz.py

@@ -1077,12 +1077,32 @@ def testGettzCacheTzLocal(self):
         local2 = tz.gettz()
 
         assert local1 is not local2
-
-    def test_gettz_same_result_for_none_and_empty_string(self):
-        local1 = tz.gettz()
-        local2 = tz.gettz("")
 
-        assert local1 == local2
+
+@pytest.mark.gettz
+def test_gettz_equal_for_empty_string_and_none():
+    local_from_none = tz.gettz()
+    local_from_empty_string = tz.gettz("")
+    assert local_from_none is not None
+    assert local_from_empty_string is not None
+    assert local_from_none == local_from_empty_string
+
+
+@pytest.mark.gettz
+@pytest.mark.parametrize("gettz_arg", [None, "", ":"]) # could add colon here
+@pytest.mark.parametrize("dt", [
+        datetime(2018, 3, 10, tzinfo=tz.gettz("America/New_York")),
+        datetime(2018, 7, 13, tzinfo=tz.gettz("Europe/London")),
+        datetime(2019, 11, 18),
+])
+def test_gettz_returns_local(gettz_arg, dt):
+    dt_act = dt.replace(tzinfo=tz.gettz())
+    dt_exp = dt.replace(tzinfo=tz.tzlocal())
+
+    assert dt_act.tzname() == dt_exp.tzname()
+    assert dt_act.utcoffset() == dt_exp.utcoffset()
+    assert dt_act.dst() == dt_exp.dst()
+    assert dt_act == dt_exp
 
 
 @pytest.mark.gettz