db/Count: print empty group if song without grouped tag exists

Be consistent with "list" responses.

src/db/Count.cxx

@@ -90,10 +90,10 @@ GroupCountVisitor(TagCountMap &map, TagType group, const LightSong &song)
 	assert(song.tag != nullptr);
 
 	const Tag &tag = *song.tag;
-	VisitTagWithFallback(tag, group,
-			     std::bind(CollectGroupCounts, std::ref(map),
-				       std::cref(tag),
-				       std::placeholders::_1));
+	VisitTagWithFallbackOrEmpty(tag, group,
+				    std::bind(CollectGroupCounts, std::ref(map),
+					      std::cref(tag),
+					      std::placeholders::_1));
 }
 
 void

src/tag/VisitFallback.hxx

@@ -49,4 +49,12 @@ VisitTagWithFallback(const Tag &tag, TagType type, F &&f) noexcept
 				    });
 }
 
+template<typename F>
+void
+VisitTagWithFallbackOrEmpty(const Tag &tag, TagType type, F &&f) noexcept
+{
+	if (!VisitTagWithFallback(tag, type, f))
+		f("");
+}
+
 #endif