A branch is just a reference to a commit. Until you commit anything to the repository, you don't have any branches. You can see this in a non-bare repository as well.
$ mkdir repo
$ cd repo
$ git init
Initialized empty Git repository in /home/me/repo/.git/
$ git branch
$ touch foo
$ git add foo
$ git commit -m "new file"
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 foo
$ git branch
* master