Distinguish empty GATT values from read failures

[vkolotov]

Summary

• treat a valid zero-length ATT Read Response as a successful read
• return null from the Java characteristic JNI only when the native read fails
• document the distinction between an empty value and a failed read

Rationale

BTGattHandler::readValue() previously returned offset > 0. That made a standards-valid empty attribute indistinguishable from a failed ATT operation and caused clients to invalidate otherwise healthy GATT state.

Validation

• built the Direct-BT fat JAR and JNI libraries successfully
• verified failed reads still take the failure path
• verified a zero-length characteristic can be read without invalidating the connection