Omit a consumer's identity where there is none

A contribution that is not a credential grant — a module offering
something to another on its own machine — has nobody to be identified
to, and was carrying an empty `as`. A field that is always present and
usually empty teaches a reader to ignore it, including when it is not.

internal/catalogue/declaration.go

@@ -396,7 +396,10 @@ type Contribution struct {
 	// used to be neither — the provisioner made a name, and the consumer, which has to present it
 	// to authenticate, had no way to learn it. One derivation reaches both ends, so they agree by
 	// construction.
-	As string `json:"as"`
+	// Omitted when there is none. A contribution that is not a credential grant — a module
+	// offering something to another on its own machine — has nobody to be identified to, and a
+	// field that is always present and usually empty teaches a reader to ignore it.
+	As string `json:"as,omitempty"`
 	// Secret is the file on this machine holding that consumer's credential, sealed to it.
 	//
 	// Named rather than carried, for the same reason the private network's key is: the mesh