Default filebeat_version fact to nil if filebeat is not installed

[willaerk]

I think the default for *_version facts is nil, if the software is not available ( eg. https://github.com/voxpupuli/puppet-php/blob/master/lib/facter/phpversion.rb). The current implementation, setting filebeat_version to false breaks spec tests for modules that include the filebeat class (a profiles module for example). This is caused by the explicit false value for the fact, as the normal fact resolution is not performed during a spec test.
Functionally, false behaves the same as the nil value, so nothing really changes by moving the value to nil if the software is not installed.