checking newNode size to make sure its not 0

[JobseRyan]

Adding a check inside NetworkClient for newNodes.size equalling 0 so that 0 isn't passed into:

int offset = this.randOffset.nextInt(newNodes.size());

resulting in logs blowing up due to "java.lang.IllegalArgumentException: bound must be positive".

For tests I have set no bootstrap urls in the client so when i try and resolve addresses none are found and thus no newNodes are found thus setting the size equal to 0 and return null after its initial exception is thrown.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[sutambe]

LGTM