Restore the constructor (empty) that I removed in d3e92fa4 because it can be...

Restore the constructor (empty) that I removed in d3e92fa4 because it can be called from subclasses

See https://github.com/piwik/piwik/commit/d3e92fa484956fd1edf853cac83e7d2c7a0f3310#commitcomment-10262413