lnrpc/watchtowerrpc: prevent watchtowerrpc panic when tower not active

lnrpc/watchtowerrpc/config_active.go

@@ -7,6 +7,9 @@ package watchtowerrpc
 // fields with struct tags are meant to parsed as normal configuration options,
 // while if able to be populated, the latter fields MUST also be specified.
 type Config struct {
+	// Active indicates if the watchtower is enabled.
+	Active bool
+
 	// Tower is the active watchtower which serves as the primary source for
 	// information presented via RPC.
 	Tower WatchtowerBackend