Null check the 'properties' element of the json message before attempting to 'rebuild' the 'DescribableList'
[FIXED JENKINS-23481][FIXED JENKINS-23551] Send onOffline notification to master computer prior Jenkins restart.

Introduces new overload `ComputerListener.onOffline(Computer, OfflineCause)`.
[JENKINS-23551] Add test
Merge pull request #1293 from olivergondza/master-listener

[FIXED JENKINS-23481][FIXED JENKINS-23551] Send onOffline notification to master computer prior Jenkins restart
Noting #1293
switched off alpha before colorizing temporary gifs when making animated balls. prevents colorize from applying fill to alpha channel. alpha is turned back on after colorize operation
Merge pull request #1280 from mattmoor/master

Fix a NPE when the 'properties' element is missing from the form's JSON
Merge pull request #1299 from kbeal/master

[JENKINS-23595] - Fix makeFlash script bug with ImageMagick v6.7.9+
Noting #1296 an #1280 (FIXED JENKINS-23437)

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>