generate_secrets: Fix handling of missing trailing newlines.
generate_secrets: Fix handling of missing trailing newlines.

When we added support for automatically adding new secrets in
generate_secrets.py, we failed to account for the possibility that a
human editor might have let the secrets file without a trailing
newline.

We address this by adding a leading newline before our new secret.

Fixes #5209.

dhcrzf/zulip,amanharitsh123/zulip,mahim97/zulip,vabs22/zulip,tommyip/zulip,jrowan/zulip,jackrzhang/zulip,jackrzhang/zulip,dhcrzf/zulip,shubhamdhama/zulip,synicalsyntax/zulip,showell/zulip,vaidap/zulip,hackerkid/zulip,Galexrt/zulip,dhcrzf/zulip,punchagan/zulip,jackrzhang/zulip,hackerkid/zulip,timabbott/zulip,rishig/zulip,zulip/zulip,Galexrt/zulip,zulip/zulip,vabs22/zulip,kou/zulip,verma-varsha/zulip,rht/zulip,hackerkid/zulip,timabbott/zulip,shubhamdhama/zulip,brockwhittaker/zulip,Galexrt/zulip,eeshangarg/zulip,vaidap/zulip,synicalsyntax/zulip,vabs22/zulip,jrowan/zulip,kou/zulip,vaidap/zulip,showell/zulip,amanharitsh123/zulip,punchagan/zulip,Galexrt/zulip,rishig/zulip,rht/zulip,kou/zulip,synicalsyntax/zulip,jackrzhang/zulip,showell/zulip,Galexrt/zulip,mahim97/zulip,timabbott/zulip,dhcrzf/zulip,shubhamdhama/zulip,timabbott/zulip,tommyip/zulip,brainwane/zulip,brockwhittaker/zulip,rishig/zulip,Galexrt/zulip,brainwane/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,brainwane/zulip,jrowan/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,jrowan/zulip,kou/zulip,punchagan/zulip,tommyip/zulip,andersk/zulip,tommyip/zulip,timabbott/zulip,punchagan/zulip,mahim97/zulip,verma-varsha/zulip,zulip/zulip,tommyip/zulip,punchagan/zulip,amanharitsh123/zulip,vaidap/zulip,Galexrt/zulip,verma-varsha/zulip,tommyip/zulip,hackerkid/zulip,vabs22/zulip,timabbott/zulip,rishig/zulip,zulip/zulip,dhcrzf/zulip,showell/zulip,hackerkid/zulip,showell/zulip,brockwhittaker/zulip,shubhamdhama/zulip,brainwane/zulip,jackrzhang/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,vabs22/zulip,verma-varsha/zulip,brainwane/zulip,rht/zulip,jrowan/zulip,rht/zulip,mahim97/zulip,rishig/zulip,brainwane/zulip,andersk/zulip,mahim97/zulip,brockwhittaker/zulip,dhcrzf/zulip,shubhamdhama/zulip,verma-varsha/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,shubhamdhama/zulip,hackerkid/zulip,amanharitsh123/zulip,vaidap/zulip,amanharitsh123/zulip,verma-varsha/zulip,showell/zulip,tommyip/zulip,vaidap/zulip,kou/zulip,mahim97/zulip,jackrzhang/zulip,eeshangarg/zulip,timabbott/zulip,rishig/zulip,zulip/zulip,vabs22/zulip,rht/zulip,zulip/zulip,punchagan/zulip,shubhamdhama/zulip,brockwhittaker/zulip,jackrzhang/zulip,synicalsyntax/zulip,rishig/zulip,rht/zulip,kou/zulip,dhcrzf/zulip,synicalsyntax/zulip,brockwhittaker/zulip,amanharitsh123/zulip,andersk/zulip,jrowan/zulip,showell/zulip,andersk/zulip