annotate command should ignore 0-sized files

[mschoettle]

In #208 it was decided to ignore 0-sized files which #205 implemented.

We are currently in the process of annotating all files in our repos using a shell script that iterates through all tracked files and calls reuse annotate --... <file>.

We ended up with __init__.py files being annotated. While we could ignore them in our shell script I wonder if annotate should raise an error or at least a warning if the file to annotate is 0-sized.