Fix nits

VCEntities/VCEntities/ResponseContaining.swift

@@ -3,7 +3,6 @@
 *  Licensed under the MIT License. See License.txt in the project root for license information.
 *--------------------------------------------------------------------------------------------*/
 
-
 public protocol ResponseContaining {
     var audienceUrl: String { get }
     var audienceDid: String { get }

VCEntities/VCEntities/presentation/PresentationResponseContainer.swift

@@ -18,8 +18,13 @@ public struct PresentationResponseContainer: ResponseContaining {
     
     public init(from presentationRequest: PresentationRequest, expiryInSeconds exp: Int = 3000) throws {
         
-        self.audienceUrl = presentationRequest.content.redirectURI ?? ""
-        self.audienceDid = presentationRequest.content.issuer ?? ""
+        guard let aud = presentationRequest.content.redirectURI,
+            let did = presentationRequest.content.issuer else {
+                throw PresentationResponseError.noAudienceSpecifiedInRequest
+        }
+        
+        self.audienceDid = did
+        self.audienceUrl = aud
         self.request = presentationRequest
         self.expiryInSeconds = exp
     }