[Bug]: elasticsearch_jvm_custom_parameters is never applied

The "Set jvm custom options" task in roles/elasticsearch/tasks/main.yml is gated by
when: elasticsearch_jvm_custom_parameters | bool, but the variable is meant to be a
list of JVM option lines (the template does {% for item in elasticsearch_jvm_custom_parameters %}).
A list is not a recognized boolean, so | bool returns False for any list — verified:
['-Xss1m'] | bool → False. The task is therefore never executed and custom JVM
options are silently ignored. No value both passes | bool and is iterable, so the
variable is effectively dead.

Fix:

• Remove elasticsearch_jvm_custom_parameters from defaults/main.yml — an optional list
  needs no default.
• In meta/argument_specs.yml: type: list, elements: str, no default.
• Change the task condition to when: elasticsearch_jvm_custom_parameters is defined
  (consistent with the other optional list variables).

To Reproduce

No response

Your Environment

• Version (commit) used:
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• Python version:
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Start with roles/elasticsearch/tasks/main.yml, defaults/main.yml, and meta/argument_specs.yml, then compare the custom-parameter handling with other optional list variables. Done means the list is accepted by the argument spec, the task is eligible when supplied, and the default no longer masks its optional nature.