ReplayGainInfo: add static method Undefined()

src/ReplayGainInfo.hxx

@@ -38,6 +38,10 @@ struct ReplayGainTuple {
 		return gain > -100;
 	}
 
+	static constexpr ReplayGainTuple Undefined() noexcept {
+		return {-200.0f, 0.0f};
+	}
+
 	gcc_pure
 	float CalculateScale(const ReplayGainConfig &config) const noexcept;
 };
@@ -49,6 +53,13 @@ struct ReplayGainInfo {
 		return track.IsDefined() || album.IsDefined();
 	}
 
+	static constexpr ReplayGainInfo Undefined() noexcept {
+		return {
+			ReplayGainTuple::Undefined(),
+			ReplayGainTuple::Undefined(),
+		};
+	}
+
 	const ReplayGainTuple &Get(ReplayGainMode mode) const noexcept {
 		return mode == ReplayGainMode::ALBUM
 			? (album.IsDefined() ? album : track)