fix: ArrayIndexOutOfBounds on empty strings

[jorsol]

1. Fix: ArrayIndexOutOfBounds on Empty Mapping

   If the mapping phase (Step 1) or Normalization phase (Step 2) resulted in an empty string (e.g., a string consisting entirely of characters mapped to "nothing"), the code would throw an ArrayIndexOutOfBoundsException. I have added a guard clause to return early if the value length is zero.

2. Optimization: Normalizer.isNormalized Guard

   To improve throughput and reduce heap churn, I've implemented a "fast-path" check using Normalizer.isNormalized. Normalizer.normalize() always allocates a new buffer/string; this is avoided if the string is already normalized.

3. Security: SecureCharBuffer Implementation

   To prevent sensitive password data from lingering in the JVM heap, the StringBuilder is replaced with a custom SecureCharBuffer. Unlike StringBuilder, this buffer implements AutoCloseable. It ensures that the internal char[] is overwritten with zeros (\0) upon closing or resizing.