Return empty list when original content is null (#1225)

When the list that can be obtained at the time of error is null, it
becomes NPE. This change returns an empty list when null, to prevent
unintended NPE.

Resolve #1223

Author: Yang-33

clients/line-bot-insight-client/src/main/java/com/linecorp/bot/insight/client/InsightClientException.java

@@ -50,6 +50,9 @@ public String getError() {
     }
 
     public List<ErrorDetail> getDetails() {
+        if (details == null) {
+            return Collections.emptyList();
+        }
         return Collections.unmodifiableList(details);
     }
 }

clients/line-bot-manage-audience-client/src/main/java/com/linecorp/bot/audience/client/ManageAudienceClientException.java

@@ -50,6 +50,9 @@ public String getError() {
     }
 
     public List<ErrorDetail> getDetails() {
+        if (details == null) {
+            return Collections.emptyList();
+        }
         return Collections.unmodifiableList(details);
     }
 }

clients/line-bot-messaging-api-client/src/main/java/com/linecorp/bot/messaging/client/MessagingApiClientException.java

@@ -57,10 +57,16 @@ public String getError() {
     }
 
     public List<ErrorDetail> getDetails() {
+        if (details == null) {
+            return Collections.emptyList();
+        }
         return Collections.unmodifiableList(details);
     }
 
     public List<SentMessage> getSentMessages() {
+        if (sentMessages == null) {
+            return Collections.emptyList();
+        }
         return Collections.unmodifiableList(sentMessages);
     }
 }