Merge bitcoin/bitcoin#22408: test: add tests for `bad-txns-prevout-null` reject reason

1f449586a9e39bc4fb53cb5c7a31362e47aea19b test: add `bad-txns-prevout-null` test to mempool_accept.py (Sebastian Falbesoner)
aa0a5bb70d77739d43d5a9ceae78fb0c6fafd435 test: add `bad-txns-prevout-null` test case to invalid_txs.py (Sebastian Falbesoner)

Pull request description:

  This simple PR adds missing tests for the reject reason `bad-txns-prevout-null`, which is thrown in the function `CheckTransaction()`: https://github.com/bitcoin/bitcoin/blob/a62fc35a150da584d39d7cd01ade14bbb5002fb9/src/consensus/tx_check.cpp#L52-L54

  Basically this condition is met for non-coinbase transactions (the code snippet above only hits if `!tx.IsCoinBase()`) with coinbase-like outpoints, i.e. hash=0, n=0xffffffff.

  Can be tested by running the functional tests `feature_block.py`, `p2p_invalid_tx.py` and `mempool_accept.py`. Not sure if the redundancy in the tests is desired (I guess it would make sense if the mempool acceptance test also makes use of the invalid_txs templates?).

ACKs for top commit:
  rajarshimaitra:
    tACK https://github.com/bitcoin/bitcoin/pull/22408/commits/1f449586a9e39bc4fb53cb5c7a31362e47aea19b
  brunoerg:
    tACK 1f449586a9e39bc4fb53cb5c7a31362e47aea19b
  kristapsk:
    ACK 1f449586a9e39bc4fb53cb5c7a31362e47aea19b, code looks correct and all tests pass.

Tree-SHA512: 2d4f940a6ac8e0d80d2670c9e1111cbf43ae6ac62809a2ccf17cffee9a41d387ea4d889ee300eb4a407c055b13bfa5d37102a32ed59964a9b6950bd907ba7204

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