Clear m_average_beam on init, pass pointer safely

!14 (merged) introduced a regression in the average beam computation.

m_average_beam was not reset in BufferSet::init anymore, resulting in a recomputation of the average beam every major cycle in wsclean. This is fixed by a m_average_beam.clear() in init.

Furthermore the m_average_beam.data() pointer was passed in an unsafe way. std::vector::data() is not guaranteed to return a null pointer, if the vector is empty. This is fixed by an check for emptiness when passing the data pointer.