This is another attempt to fix#13001 on `branch-5.1`.
In #13001 we found a test case which causes a crash on `branch-5.1` because it didn't handle `UNSET_VALUE` properly:
```python3
def test_unset_insert_where(cql, table2):
p = unique_key_int()
stmt = cql.prepare(f'INSERT INTO {table2} (p, c) VALUES ({p}, ?)')
with pytest.raises(InvalidRequest, match="unset"):
cql.execute(stmt, [UNSET_VALUE])
def test_unset_insert_where_lwt(cql, table2):
p = unique_key_int()
stmt = cql.prepare(f'INSERT INTO {table2} (p, c) VALUES ({p}, ?) IF NOT EXISTS')
with pytest.raises(InvalidRequest, match="unset"):
cql.execute(stmt, [UNSET_VALUE])
```
This problem has been fixed on `master` by PR #12517. I tried to backport it to `branch-5.1` (#13029), but this didn't go well - it was a big change that touched a lot of components. It's hard to make sure that it won't cause some unexpected issues.
Then I made a simpler fix for `branch-5.1`, which achieves the same effect as the original PR (#13057).
The problem is that this effect includes backwards incompatible changes - it bans UNSET_VALUE in some places that `branch-5.1` used to allow.
Breaking changes are bad, so I made this PR, which does an absolutely minimal change to fix the crash.
It adds a check the moment before the crash would happen.
To make sure that everything works correctly, and to detect any possible breaking changes, I wrote a bunch of tests that validate the current behavior.
I also ported some tests from the `master` branch, at least the ones that were in line with the behavior on `branch-5.1`.
Closes#13133
* github.com:scylladb/scylladb:
cql-pytest/test_unset: port some tests from master branch
cql-pytest/test_unset: test unset value in UPDATEs with LWT conditions
cql-pytest/test_unset: test unset value in UPDATEs with IF EXISTS
cql-pytest/test_unset: test unset value in UPDATE statements
cql-pytest/test_unset: test unset value in INSERTs with IF NOT EXISTS
cql-pytest/test_unset: test unset value in INSERT statements
cas_request: fix crash on unset value in primary key with LWT
7007c94f5d