Minor improvements in Jenkins Node-related classes.
The change adds some missing null checks

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Added missing null checks for Computer::getNode(), SlaveComputer::getNode() and AbstractBuild::getCurrentNode()

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

Conflicts:
	core/src/main/java/hudson/model/AbstractBuild.java
[nodes handling] - Fixed code according to comments from @jglick

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

Conflicts:
	core/src/main/java/hudson/model/Job.java
[node null checks] - Throw the HTTP "Not found exception"

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Fixed node handling methods in order to comply with https://github.com/jenkinsci/jenkins/commit/3f8fc4e2ec5b1b421e1551a2a2fc2a284be330d1

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge pull request #1213 from synopsys-arc-oss/node_classes_improvements

[READY] Additional null checks for node access methods.