fix a potential bug if a case block doesn't have a date_modified property
fix a potential bug if a case block doesn't have a date_modified property

puttarajubr/commcare-hq,qedsoftware/commcare-hq,SEL-Columbia/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq,qedsoftware/commcare-hq,SEL-Columbia/commcare-hq,puttarajubr/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq,puttarajubr/commcare-hq,puttarajubr/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq,SEL-Columbia/commcare-hq