bitbucket: Fix support for pushes that update a tag.
bitbucket: Fix support for pushes that update a tag.

Previously, we threw an exception if created/closed were both unset;
apparently that can happen when pushing an update to a tag.

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