[FIXES JENKINS-14321] - Do not re-use last build's environment for remote polling

Fixes, the original 'fix' in 3d9a73d26b129249c51fccfca6060280197b25eb it appears
that a crucial check on the newly implemented argument wasn't actually implemented.

I've changed the condition to test this rather than set 'b' to null if reuseLastBuildEnv
is set to true as 'b' is de-referenced later in the getPollEnvironment method.

Rather worryingly it appears that the existing code in getPollEnvironment tests at one
point whether 'b' is null, but then later (outside this test) derefences it anyway, but
this commit does not make that particular problem any worse.

Signed-off-by: ciaranj <ciaranj@gmail.com>
Fixes potential NPE if no previous build exists

Signed-off-by: ciaranj <ciaran.jessup@thisisnumero.com>