If a profile has a data files directory that looks like this:
```
files/platforms/one/data.json
files/platforms/two/data.json
files/platforms/three/data.json
```
... the source reader will return the directories in the list of files but with
nil contents. This causes an issue when Inspec::Profile tries to create a sha256
checksum of the profile contents only to try to cast nil to a string when
building the null-delimited profile contents string.
Files that are empty will have an empty string as its contents, so it's safe to
assume that file entries with nil contents are actually a directory and have no
affect on the profile's checksum. Therefore, this change will eliminate any file
entries in responses from the source readers where the contents are nil.
Signed-off-by: Adam Leff <adam@leff.co>
