slack import: Fix handling of tombstone files.
slack import: Fix handling of tombstone files.

Apparently, the mode attribute is not always present.

punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,brainwane/zulip,zulip/zulip,synicalsyntax/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,rishig/zulip,brainwane/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,shubhamdhama/zulip,eeshangarg/zulip,shubhamdhama/zulip,eeshangarg/zulip,rht/zulip,tommyip/zulip,andersk/zulip,tommyip/zulip,shubhamdhama/zulip,timabbott/zulip,tommyip/zulip,tommyip/zulip,andersk/zulip,rishig/zulip,brainwane/zulip,punchagan/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rishig/zulip,shubhamdhama/zulip,timabbott/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,kou/zulip,tommyip/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,rishig/zulip,rht/zulip,tommyip/zulip,punchagan/zulip,kou/zulip,showell/zulip,hackerkid/zulip,punchagan/zulip,shubhamdhama/zulip,zulip/zulip,punchagan/zulip,synicalsyntax/zulip,kou/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,showell/zulip,showell/zulip,timabbott/zulip,timabbott/zulip,rishig/zulip,timabbott/zulip,showell/zulip,rishig/zulip,rht/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,timabbott/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,hackerkid/zulip,synicalsyntax/zulip,rishig/zulip,showell/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,tommyip/zulip