Handle empty JVM_OPTIONS correctly on dynamic/container agents

[Copilot]

Fixes: adoptium/TKG#862
Fixes: #6835

System tests on dynamic Azure agents were not honoring playlist variation JVM args because containerized runs propagated JVM_OPTIONS="" into the shell, which suppresses Makefile defaults defined via JVM_OPTIONS?=.

• Problem context

  • Static-node runs effectively had JVM_OPTIONS unset, so variation-derived options were applied.
  • Dynamic/container runs had JVM_OPTIONS explicitly set to empty, causing an unintended empty override.

• Pipeline behavior update (buildenv/jenkins/JenkinsfileBase)

  • In makeTest(...), conditionally prepend unset JVM_OPTIONS; before invoking make when env.JVM_OPTIONS is null/empty/whitespace.
  • In genParallelList(...), apply the same conditional unset JVM_OPTIONS; before make genParallelList for consistency in dynamic parallel flows.

• Behavioral impact

  • Empty/unset job parameter now behaves as “no override,” allowing playlist variation args to flow through.
  • Non-empty JVM_OPTIONS continues to intentionally override variation/default options.

if (!env.JVM_OPTIONS?.trim()) {
    makeTestCmd = "unset JVM_OPTIONS; $makeTestCmd"
}