OPENNLP-1916: minor normalizer fix

[krickert]

1. Minor: Replace the rung metaphor with "normalizer" wording.
2. Minor:
   Validate and copy the aggregate normalizer chain at construction: AggregateCharSequenceNormalizer stored the caller's varargs array as-is (null array or null element caused NPE mid-normalize, which cascaded into a changed aggregate).

I changed it to fail loudly.

Now throws IllegalArgumentException on null, defensive copy, shadow gone, constructor javadoc added, plus a new 4-test AggregateCharSequenceNormalizerTest covering order, both null rejections, and array-mutation isolation.

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