fix(dav): don't crash subscription on invalid calendar object #12206

Job	Run time
changes	6s
integration-sqlite (capabilities_features, 8.1, main)	2m 30s
integration-sqlite (collaboration_features, 8.1, main)	3m 30s
integration-sqlite (comments_features, 8.1, main)	3m 16s
integration-sqlite (dav_features, 8.1, main)	6m 33s
integration-sqlite (features, 8.1, main)	9m 45s
integration-sqlite (federation_features, 8.1, main)	5m 14s
integration-sqlite (--tags ~@large files_features, 8.1, main)	11m 20s
integration-sqlite (filesdrop_features, 8.1, main)	2m 50s
integration-sqlite (openldap_features, 8.1, main)	2m 53s
integration-sqlite (openldap_numerical_features, 8.1, main)	2m 38s
integration-sqlite (ldap_features, 8.1, main)	3m 12s
integration-sqlite (remoteapi_features, 8.1, main)	2m 6s
integration-sqlite (setup_features, 8.1, main)	2m 22s
integration-sqlite (sharees_features, 8.1, main)	5m 36s
integration-sqlite (sharing_features, 8.1, main)	14m 6s
integration-sqlite (videoverification_features, 8.1, main)	6m 56s
integration-sqlite-summary	4s
	1h 24m 57s

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