protect against empty contents

Fetching the content of an empty file through the API result into out of bounds exception:

```
java.lang.ArrayIndexOutOfBoundsException: -1
	at clojure.data.codec.base64$pad_length.invokePrim(base64.clj:53)
	at clojure.data.codec.base64$decode.invokePrim(base64.clj:146)
	at clojure.data.codec.base64$decode.invoke(base64.clj:144)
	at tentacles.repos$decode_b64.invoke(repos.clj:417)
	at tentacles.repos$decode_b64.invoke(repos.clj:422)
	at tentacles.repos$contents.invoke(repos.clj:442)
```