[READY][FIXED JENKINS-15094] - Handle null parameter values to avoid massive executor deaths

The change adds missing checks for null <code>ParameterValue</code>, which may be passed from the CLI.

It prevents the death of Jenkins executors during the <code>canTake()</code> check of node labels. Such issue leads to the complete DoS, because Jenkins tries another executor if NPE occurs :( 

Tests will be added soon...

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>