Callback type: Make value optional, make error nullable

[thomastoye]

Without this, code like the following will fail to compile with strictNullChecks on:

import avsc from 'avsc'
import crc32 from 'buffer-crc32'
import snappy from 'snappy'

export const encodeAvro = (schema: avsc.Schema) =>
  new avsc.streams.BlockEncoder(schema, {
    writeHeader: 'auto',
    codec: 'snappy',
    codecs: {
      snappy: (buf, cb) => {
        const checksum = crc32(buf)
        snappy.compress(buf, (err, deflated) => {
          if (err) {
            return cb(err) // <---
          }

          const block = Buffer.alloc(deflated.length + 4)
          deflated.copy(block)
          checksum.copy(block, deflated.length)
          cb(null, block)
        })
      }
    }
  })

The compiler will complain that no argument is given for value in cb(err).

Example from a callback type in @types/node for reference: type TransformCallback = (error?: Error | null, data?: any) => void;

[coveralls]

Coverage remained the same at 99.676% when pulling 6c08569 on thomastoye:patch-1 into ac8d040 on mtth:master.

[mtth]

Thanks for the catch and fix @thomastoye.