[JENKINS-15094] - Unit tests for the issue

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
[FIXED JENKINS-15094] - Handle null parameter values in BuildCommand (CLI) and Parameter classes (CLI).

The change adds missing checks for null ParameterValue, which may be passed from the CLI.
It prevents the death of Jenkins executors during the canTake() check of node labels.

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Javadoc: Noting changes in Executor availability check approaches in 1.536

This change is a follow-up to Executor::isAlive() issues in #1229.

Since 1.536, {@link Executor}s start threads on-demand.
The entire logic should use Executor::isActive() instead of Executor::isAlive() in order to check if the {@link Executor} it ready to take tasks.

Since the change breaks previous executor availability flows, it is required to document changes somehow.

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge pull request #1262 from synopsys-arc-oss/javadoc_hudson_model_executor_isActive

Javadoc: Noting changes in Executor availability check approaches (1.536)
Merge pull request #1229 from synopsys-arc-oss/CLI_null_ParameteValue

[READY][FIXED JENKINS-15094] - Handle null parameter values to avoid massive executor deaths
Refactor BuildCommandTest
Noting #1229