util/{Const,Writable}Buffer, ...: rename IsEmpty() to empty(), imitating STL

src/tag/Builder.cxx

@@ -191,7 +191,7 @@ TagBuilder::Complement(const Tag &other)
 inline void
 TagBuilder::AddItemInternal(TagType type, StringView value)
 {
-	assert(!value.IsEmpty());
+	assert(!value.empty());
 
 	auto f = FixTagString(value);
 	if (!f.IsNull())
@@ -209,7 +209,7 @@ TagBuilder::AddItemInternal(TagType type, StringView value)
 void
 TagBuilder::AddItem(TagType type, StringView value)
 {
-	if (value.IsEmpty() || !IsTagEnabled(type))
+	if (value.empty() || !IsTagEnabled(type))
 		return;
 
 	AddItemInternal(type, value);

src/tag/Builder.hxx

@@ -74,7 +74,7 @@ public:
 	 * Returns true if the tag contains no items.  This ignores
 	 * the "duration" attribute.
 	 */
-	bool IsEmpty() const {
+	bool empty() const {
 		return items.empty();
 	}
 
@@ -83,7 +83,7 @@ public:
 	 */
 	gcc_pure
 	bool IsDefined() const noexcept {
-		return !duration.IsNegative() || has_playlist || !IsEmpty();
+		return !duration.IsNegative() || has_playlist || !empty();
 	}
 
 	void Clear();

src/tag/Id3Scan.cxx

@@ -334,7 +334,7 @@ tag_id3_import(struct id3_tag *tag)
 {
 	TagBuilder tag_builder;
 	scan_id3_tag(tag, add_tag_handler, &tag_builder);
-	return tag_builder.IsEmpty()
+	return tag_builder.empty()
 		? nullptr
 		: tag_builder.CommitNew();
 }