Don't fail a test because there are no public IP addresses

When running the test suite on a machine without public IP addresses
there is a list index out of range error:

=================================== FAILURES ===================================
_____________________________ test_disambiguate_ip _____________________________
    def test_disambiguate_ip():
        # garbage in, garbage out
>       public_ip = public_ips()[0]
E       IndexError: list index out of range
ipyparallel/tests/test_util.py:11: IndexError

This commit modifies the test by adding a check that the list is not
empty before trying to access its first element.

Author: ellert

ipyparallel/tests/test_util.py

@@ -8,12 +8,13 @@
 
 def test_disambiguate_ip():
     # garbage in, garbage out
-    public_ip = public_ips()[0]
     assert util.disambiguate_ip_address('garbage') == 'garbage'
     assert util.disambiguate_ip_address('0.0.0.0', socket.gethostname()) == localhost()
     wontresolve = 'this.wontresolve.dns'
     with pytest.warns(
         RuntimeWarning, match=f"IPython could not determine IPs for {wontresolve}"
     ):
         assert util.disambiguate_ip_address('0.0.0.0', wontresolve) == wontresolve
-    assert util.disambiguate_ip_address('0.0.0.0', public_ip) == localhost()
+    if public_ips():
+        public_ip = public_ips()[0]
+        assert util.disambiguate_ip_address('0.0.0.0', public_ip) == localhost()