Fix from Brian for checkstyle rule with no sources

Change-Id: I37a85b38b61d9b008dc3456345bbd9ded6718281

@@ -109,6 +109,7 @@ def osgi_jar(
   )
 
   ### Checkstyle
+  if srcs:
       chk_cmd = '#!/bin/sh\n'
       base = get_base_path()
       chk_cmd += ' '.join(( 'java -jar $(location //lib:checkstyle)',
@@ -130,6 +131,8 @@ def osgi_jar(
         deps = [ ':'+ bare_jar_name ],
         labels = [ 'checkstyle' ],
       )
+  else:
+      print 'Not generating checkstyle rule for %s because there are no sources.' % name
 
   # TODO add project config for intellij
   # project_config(