Cannot parse `://a`

```clojure
user=> (fast-edn.core/read-string "://a")
Execution error at fast_edn.EdnParser/finalizeKeyword (EdnParser.java:632).
Keyword's namespace can't be empty: //a, offset: 4, context:
://a
   ^, offset: 4, context:
://a
   ^

user=> (clojure.edn/read-string "://a")
://a
```