fix LooseVersion exception with empty distribution_version

library/system/hostname

@@ -299,7 +299,8 @@ class RedHat5Hostname(Hostname):
 class RedHatServerHostname(Hostname):
     platform = 'Linux'
     distribution = 'Red hat enterprise linux server'
-    if LooseVersion(get_distribution_version()) >= LooseVersion("7"):
+    distribution_version = get_distribution_version()
+    if distribution_version and LooseVersion(distribution_version) >= LooseVersion("7"):
         strategy_class = FedoraStrategy
     else:
         strategy_class = RedHatStrategy
@@ -307,7 +308,8 @@ class RedHatServerHostname(Hostname):
 class RedHatWorkstationHostname(Hostname):
     platform = 'Linux'
     distribution = 'Red hat enterprise linux workstation'
-    if LooseVersion(get_distribution_version()) >= LooseVersion("7"):
+    distribution_version = get_distribution_version()
+    if distribution_version and LooseVersion(distribution_version) >= LooseVersion("7"):
         strategy_class = FedoraStrategy
     else:
         strategy_class = RedHatStrategy