kiji-schema: SCHEMA-590. AvroCellEncoder should check for null pointer writer schema

 - AvroCellEncoder now throws KijiEncodingException if no writer schema
   can be inferred from the value to encode.

issue: https://jira.kiji.org/browse/SCHEMA-590
review: https://review.kiji.org/r/1286
commit: https://github.com/kijiproject/kiji-schema/commit/cae998cee80c42c24df6c96c6a6f7ebd64d47a79