The existing logic in `#audit_head_branch` for identifying the `HEAD`
branch in a Git repository will give an ```undefined method `[]' for
nil:NilClass``` error when a repository doesn't provide this
reference. Expected output is as follows:
```
ref: refs/heads/master HEAD
1a8f9ac700873d1a08de31a17a2fd654245d5085 HEAD
```
However, I encountered this error for a repository with the following
output (i.e., where no symref is provided for HEAD):
```
f86be659718c0cd0a67f88b42f07044c23d0d028 HEAD
```
This commit resolves the error by modifying the related logic to
account for a `nil` value.
