Enforce naming conventions and allow empty (except for the header) types.hal files.
diff --git a/hidl-gen_y.yy b/hidl-gen_y.yy
index 93e7b0b..e1d46ce 100644
--- a/hidl-gen_y.yy
+++ b/hidl-gen_y.yy
@@ -186,7 +186,7 @@
     ;
 
 type_declarations
-    : type_declaration
+    : /* empty */
     | type_declarations type_declaration
     ;