Added property test for tz.gettz()

The current tests only check that `tz.gettz()` and `tz.gettz("")` do
the same thing, but not that the thing that they do is equivalent to a
local time zone.

Defining what is a "local time" is a bit complicated, considering that
`tz.gettz()` may return a `tzfile` or a `tzlocal`, and neither of those
have the same behavior as Python's built-in local time support via
`.astimezone()`. This property test checks only the datetimes and
properties that are expected to be the same between the various methods
of getting a "local" time zone.

dateutil/test/property/test_tz_prop.py

@@ -0,0 +1,35 @@
+from datetime import datetime, timedelta
+
+import pytest
+import six
+from hypothesis import assume, given
+from hypothesis import strategies as st
+
+from dateutil import tz as tz
+
+EPOCHALYPSE = datetime.fromtimestamp(2147483647)
+NEGATIVE_EPOCHALYPSE = datetime.fromtimestamp(0) - timedelta(seconds=2147483648)
+
+
+@pytest.mark.gettz
+@pytest.mark.parametrize("gettz_arg", [None, ""])
+# TODO: Remove bounds when GH #590 is resolved
+@given(
+    dt=st.datetimes(
+        min_value=NEGATIVE_EPOCHALYPSE, max_value=EPOCHALYPSE, timezones=st.just(tz.UTC),
+    )
+)
+def test_gettz_returns_local(gettz_arg, dt):
+    act_tz = tz.gettz(gettz_arg)
+    if isinstance(act_tz, tz.tzlocal):
+        return
+
+    dt_act = dt.astimezone(tz.gettz(gettz_arg))
+    if six.PY2:
+        dt_exp = dt.astimezone(tz.tzlocal())
+    else:
+        dt_exp = dt.astimezone()
+
+    assert dt_act == dt_exp
+    assert dt_act.tzname() == dt_exp.tzname()
+    assert dt_act.utcoffset() == dt_exp.utcoffset()

dateutil/tz/tz.py

@@ -1596,7 +1596,7 @@ def nocache(name=None):
                     name = os.environ["TZ"]
                 except KeyError:
                     pass
-            if not name or name == ":":
+            if name is None or name in ("", ":"):
                 for filepath in TZFILES:
                     if not os.path.isabs(filepath):
                         filename = filepath