Bug 1931475 - Fix BUSY status in caldav calendar FreeBusy. r=leftmostcat

Differential Revision: https://phabricator.services.mozilla.com/D230071

--HG--
extra : amend_source : 22f4861a058e9a579a6665ac0522b57b876cf80f

calendar/providers/caldav/modules/CalDavRequest.sys.mjs

@@ -1130,7 +1130,9 @@ class FreeBusyResponse extends CalDavSimpleResponse {
             }
 
             for (const fbprop of cal.iterate.icalProperty(fbcomp, "FREEBUSY")) {
-              const type = fbprop.getParameter("FBTYPE");
+              // According to https://datatracker.ietf.org/doc/html/rfc5545#section-3.2.9,
+              // the default value for the FBTYPE parameter is BUSY if not explicitly specified.
+              const type = fbprop.getParameter("FBTYPE") || "BUSY";
 
               const parts = fbprop.value.split("/");
               const begin = cal.createDateTime(parts[0]);