java: add explicit null check in BrotliInputStream.read

[leofernandesmo]

• Problem: BrotliInputStream.read(byte[], int, int) currently dereferences destBuffer.length during bounds checking, so passing a null buffer triggers a NullPointerException from the dereference rather than an explicit parameter check.

• Why change: InputStream.read(byte[], int, int) specifies NullPointerException when the buffer argument is null, so this change preserves the expected exception type while making the failure mode explicit and easier to diagnose.

Repro:

InputStream in = InputStream.nullInputStream();
BrotliInputStream br = new BrotliInputStream(in);
br.read((byte[]) null, 0, 1);

• Fix: add early destBuffer == null check and throw NullPointerException("destBuffer is null").
• CLA note: “I understand this repo requires the Google CLA and will complete it if/when requested.”

[google-cla]

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[leofernandesmo]

I already signed the CLA