[Audio Converter] Minor change for format changes

This should also seal up any potential hole for problems if there's an
audio format change and no audio buffered.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Audio/Chain/ConverterNode.m

@@ -710,6 +710,7 @@ tryagain:
 
 	ioNumberPackets -= ioNumberPackets % dmFloatFormat.mBytesPerPacket;
 
+	if(ioNumberPackets) {
 		AudioChunk *chunk = [[AudioChunk alloc] init];
 		[chunk setFormat:nodeFormat];
 		if(nodeChannelConfig) {
@@ -719,9 +720,12 @@ tryagain:
 
 		floatOffset += ioNumberPackets;
 		amountRead += ioNumberPackets;
-
 		convertEntered = NO;
 		return chunk;
+	}
+
+	convertEntered = NO;
+	return nil;
 }
 
 - (void)observeValueForKeyPath:(NSString *)keyPath