probe: Fixed xmp_chapters value if no chapters are found

probe.c

@@ -28,20 +28,20 @@ char *get_xmpchapters(char *xmp)
 	xmlNodePtr node;
 	double framerate, start;
 	if (!(doc = xmlParseMemory(xmp, strlen(xmp))))
-		return "";
+		return "[]";
 	if (!(xpath = xmlXPathNewContext(doc)))
-		return "";
+		return "[]";
 	xmlXPathRegisterNs(xpath, "x", "adobe:ns:meta/");
 	xmlXPathRegisterNs(xpath, "rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#");
 	xmlXPathRegisterNs(xpath, "xmpDM", "http://ns.adobe.com/xmp/1.0/DynamicMedia/");
 	res = xmlXPathEvalExpression("/x:xmpmeta/rdf:RDF/rdf:Description/@xmpDM:videoFrameRate", xpath);
 	if (!res || !res->nodesetval->nodeNr)
-		return "";
+		return "[]";
 	framerate = strtod(res->nodesetval->nodeTab[0]->children->content, 0);
 	xmlXPathFreeObject(res);
 	res = xmlXPathEvalExpression("/x:xmpmeta/rdf:RDF/rdf:Description/xmpDM:Tracks/rdf:Bag/rdf:li/rdf:Description/xmpDM:markers/rdf:Seq/rdf:li/rdf:Description", xpath);
 	if (!res || !res->nodesetval)
-		return "";
+		return "[]";
 	ptr = 0;
 	size = 0;
 	if (!(stream = open_memstream(&ptr, &size)))