Fixes a panic on writes because the field value was not parse correctly.
panic: unsupported value type during encode fields: <nil>
goroutine 117 [running]:
github.com/influxdb/influxdb/tsdb.(*FieldCodec).EncodeFields(0xc2081c4020, 0xc2081dc180, 0x0, 0x0, 0x0, 0x0, 0x0)
/Users/jason/go/src/github.com/influxdb/influxdb/tsdb/shard.go:573 +0x8e3