Handle missing compression block size

Reader.cc (lines 59-65) handles when the writer doesn't set compressionBlockSize in the postscript by checking if it's set and using 256 * 1024 if it's not.

The ReaderImpl.java does no such checks (lines 744 and 826), and uses the protobuffer "empty" value of 0 when the value is not specified.

This means that valid files (compressionBlockSize is optional after all) can't be read by the Java implementation, but can be by the C++ implementation.