Do not try to process an empty metai/main.yml file

If an empty meta/main.yml file exists, ansible-galaxy threw an unhelpful exception. Now, a warning message is printed and the installation does not fail.

See https://stackoverflow.com/questions/45432994/ansible-galaxy-fails-on-dependency-with-empty-meta-main-yml

lib/ansible/cli/galaxy.py

@@ -397,6 +397,9 @@ class GalaxyCLI(CLI):
 
             # install dependencies, if we want them
             if not no_deps and installed:
+                if not role.metadata:
+                    display.warning("Meta file %s is empty. Skipping dependencies." % role.path)
+                else:
                     role_dependencies = role.metadata.get('dependencies') or []
                     for dep in role_dependencies:
                         display.debug('Installing dep %s' % dep)