Make MessageContent::Choose() return an optional

And propagate this through the File, Message and PluginCleaningData APIs
that call it.

include/loot/metadata/file.h

@@ -105,7 +105,8 @@ public:
    * @return The MessageContent object for the preferred language, or if one
    *         does not exist, the English-language MessageContent object.
    */
-  LOOT_API MessageContent ChooseDetail(const std::string& language) const;
+  LOOT_API std::optional<MessageContent> ChooseDetail(
+      const std::string& language) const;
 
 private:
   Filename name_;

include/loot/metadata/message.h

@@ -109,7 +109,7 @@ public:
    * @return A MessageContent object for the preferred language, or for English
    *         if a MessageContent object is not available for the given language.
    */
-  LOOT_API MessageContent GetContent(const std::string& language) const;
+  LOOT_API std::optional<MessageContent> GetContent(const std::string& language) const;
 
   /**
    * Get the message as a SimpleMessage given a language.
@@ -118,7 +118,7 @@ public:
    * @return A SimpleMessage object for the preferred language, or for English
    *         if message text is not available for the given language.
    */
-  LOOT_API SimpleMessage ToSimpleMessage(const std::string& language) const;
+  LOOT_API std::optional<SimpleMessage> ToSimpleMessage(const std::string& language) const;
 
 private:
   MessageType type_;
@@ -133,7 +133,7 @@ LOOT_API bool operator!=(const Message& lhs, const Message& rhs);
 
 /**
  * Check if the first Message object is greater than the second Message object.
- * @returns True if the second Message object is less than the first Message 
+ * @returns True if the second Message object is less than the first Message
  *          object, false otherwise.
  */
 LOOT_API bool operator>(const Message& lhs, const Message& rhs);
@@ -141,13 +141,13 @@ LOOT_API bool operator>(const Message& lhs, const Message& rhs);
 /**
  * Check if the first Message object is less than or equal to the second
  * Message object.
- * @returns True if the first Message object is not greater than the second 
+ * @returns True if the first Message object is not greater than the second
  *          Message object, false otherwise.
  */
 LOOT_API bool operator<=(const Message& lhs, const Message& rhs);
 
 /**
- * Check if the first Message object is greater than or equal to the second 
+ * Check if the first Message object is greater than or equal to the second
  * Message object.
  * @returns True if the first Message object is not less than the second
  *          Message object, false otherwise.

include/loot/metadata/message_content.h

@@ -26,6 +26,7 @@
 
 #include <string>
 #include <vector>
+#include <optional>
 
 #include "loot/api_decorator.h"
 
@@ -108,9 +109,9 @@ public:
    *         If no locale or language code matches are found and content in the
    *         default language is present, that content is returned.
    *
-   *         Otherwise, a default-constructed MessageContent is returned.
+   *         Otherwise, an empty optional is returned.
    */
-  LOOT_API static MessageContent Choose(
+  LOOT_API static std::optional<MessageContent> Choose(
       const std::vector<MessageContent> content,
       const std::string& language);
 

include/loot/metadata/plugin_cleaning_data.h

@@ -144,7 +144,7 @@ public:
    * @return The MessageContent object for the preferred language, or if one
    *         does not exist, the English-language MessageContent object.
    */
-  LOOT_API MessageContent ChooseDetail(const std::string& language) const;
+  LOOT_API std::optional<MessageContent> ChooseDetail(const std::string& language) const;
 
 private:
   uint32_t crc_;