reactor: ensure glob_ref is a string

When path does not exist, cp.cache_file returns False, which will cause
a traceback when we attempt to glob that path.

salt/utils/reactor.py

@@ -58,7 +58,7 @@ def render_reaction(self, glob_ref, tag, data):
         react = {}
 
         if glob_ref.startswith('salt://'):
-            glob_ref = self.minion.functions['cp.cache_file'](glob_ref)
+            glob_ref = self.minion.functions['cp.cache_file'](glob_ref) or ''
         globbed_ref = glob.glob(glob_ref)
         if not globbed_ref:
             log.error('Can not render SLS {0} for tag {1}. File missing or not found.'.format(glob_ref, tag))