fix: ArrayIndexOutOfBoundsException on empty string after mapping

[jorsol]

• fix: ArrayIndexOutOfBoundsException on empty string after 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.
  Now there is a guard clause to return early if the value length is zero.

• fix: add Normalizer.isNormalized guard to avoid allocations
  Improve throughput and reduce heap churn, use a "fast-path" check using
  Normalizer.isNormalized. Normalizer.normalize() always allocates a new
  buffer/string; this is avoided if the string is already normalized.

• feat: add SecureStringBuilder for memory-safe string building
  Introduced SecureStringBuilder to handle sensitive data during
  stringprep operations. Unlike standard StringBuilder, this
  implementation ensures internal buffers are zeroed out on resize and
  closure to mitigate memory exposure of cryptographic material.

• chore: replace individual boolean flags with EnumSet from profile
  Refactored Stringprep to store the EnumSet directly instead of unpacking
  it into 18 individual boolean flags. This eliminates constructor
  boilerplate, reduces class bloat, and simplifies the internal state.