RuntimeError if submodule has no branch

If a submodule has been added without the --branch option, accessing the Submodule.branch property raises a RuntimeError:

❯❯❯ git init
Initialized empty Git repository in /tmp/gregoire/tmp.R3p5f9ZTwD/.git/
❯❯❯ git submodule add https://github.com/libgit2/pygit2.git pygit2  --branch=master          
[...]
❯❯❯ python3 -c 'import pygit2; print(pygit2.Repository(".").lookup_submodule("pygit2").branch)'          
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/gregoire/.local/lib/python3.7/site-packages/pygit2/submodule.py", line 80, in branch
    return ffi.string(branch).decode('utf-8')
RuntimeError: cannot use string() on <cdata 'char *' NULL>

It works fine if --branch is added to git submodule add:

❯❯❯ git init
Initialized empty Git repository in /tmp/gregoire/tmp.R3p5f9ZTwD/.git/
❯❯❯ git submodule add https://github.com/libgit2/pygit2.git pygit2
[...]
❯❯❯ python3 -c 'import pygit2; print(pygit2.Repository(".").lookup_submodule("pygit2").branch)'
master

Start at the Submodule.branch property in pygit2/submodule.py, corresponding to the failing line shown in the report, and reproduce the case with a submodule added without --branch. Add regression coverage for that case and verify that accessing branch no longer raises the NULL cdata RuntimeError while matching the intended API behavior.