restore empty message to fix broken hyperlinks

Signed-off-by: dhh1128 <daniel.hardman@gmail.com>

docs/spec-files/empty.md

@@ -6,18 +6,6 @@ The [PIURI](#protocol-identifier-uri) for this protocol is:
 
     https://didcomm.org/empty/1.0
 
-#### Roles
-
-There are two parties in a trust ping: the `sender`
-and the `receiver`. The sender initiates the trust
-ping. The receiver responds. If the receiver wants
-to do a ping of their own, they can, but this is a
-new interaction in which they become the sender.
-
-#### Messages
-
-##### empty
-
 The `empty` message has no semantic meaning. The message's only purpose is to allow the transfer of message headers. 
 
 ```JSON

specs.json

@@ -29,6 +29,7 @@
         "routing.md",
         "out_of_band.md",
         "feature_discovery.md",
+        "empty.md",
         "trustping.md",
         "i18n.md",
         "future_proofing.md",