RFR: 8366454: TLS1.3 server fails with bad_record_mac when receiving encrypted records with empty body

[Alice Pellegrini]

According to RFC 8446 section 5.4, third paragraph 
> Application Data records may contain a zero-length
>    TLSInnerPlaintext.content if the sender desires.  This permits
>    generation of plausibly sized cover traffic in contexts where the
>    presence or absence of activity may be sensitive.  Implementations
>    MUST NOT send Handshake and Alert records that have a zero-length
>    TLSInnerPlaintext.content; if such a message is received, the
>    receiving implementation MUST terminate the connection with an
>    "unexpected_message" alert.


The proposed change removes an off by 1 error in the SSLCipher implementation, forces the correct Alert message to be sent in response to zero-length Alert fragments, as well as updating some tests which detected the BadPaddingException but now detect a SSLProtocolException, which is thrown by `TransportContext.fatal`

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Commit messages:
 - clearer error message and test fix
 - 8366454: TLS1.3 server fails with bad_record_mac when receiving encrypted records with empty body

Changes: https://git.openjdk.org/jdk/pull/27438/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27438&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366454
  Stats: 17 lines in 4 files changed: 10 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/27438.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27438/head:pull/27438

PR: https://git.openjdk.org/jdk/pull/27438