Fixed serializing of exceptions without message
Refactored
SuiteEventSerializer: explicit operations for reading/writing nullable strings
Refactored: renamed StubByteBufferSequence to FixedByteBufferSequence, tested it
ByteBufferSequenceContract: do not let the position/limit be modified
SuiteEventSerializerTest: use a FixedByteBufferSequence to avoid timeouts in PIT when e.g. a writeString is removed