TagBuilder: add method AddEmptyItem()

src/tag/TagBuilder.cxx

@@ -219,6 +219,16 @@ TagBuilder::AddItem(TagType type, const char *value)
 }
 
 void
+TagBuilder::AddEmptyItem(TagType type)
+{
+	tag_pool_lock.lock();
+	auto i = tag_pool_get_item(type, "", 0);
+	tag_pool_lock.unlock();
+
+	items.push_back(i);
+}
+
+void
 TagBuilder::RemoveAll()
 {
 	tag_pool_lock.lock();

src/tag/TagBuilder.hxx

@@ -154,6 +154,13 @@ public:
 	void AddItem(TagType type, const char *value);
 
 	/**
+	 * Appends a new tag item with an empty value.  Do not use
+	 * this unless you know what you're doing - because usually,
+	 * empty values are discarded.
+	 */
+	void AddEmptyItem(TagType type);
+
+	/**
 	 * Removes all tag items.
 	 */
 	void RemoveAll();