LWT queries with empty clustering range used to cause a crash.
For example in:
```cql
UPDATE tab SET r = 9000 WHERE p = 1 AND c = 2 AND c = 2000 IF r = 3
```
The range of `c` is empty - there are no valid values.
This caused a segfault when accessing the `first` range:
```c++
op.ranges.front()
```
Cassandra rejects such queries at the preparation stage. It doesn't allow two `EQ` restriction on the same clustering column when an IF is involved.
We reject them during runtime, which is a worse solution. The user can prepare a query with `c = ? AND c = ?`, and then run it, but unexpectedly it will throw an `invalid_request_exception` when the two bound variables are different.
We could ban such queries as well, we already ban the usage of `IN` in conditional statements. The problem is that this would be a breaking change.
A better solution would be to allow empty ranges in `LWT` statements. When an empty range is detected we just wouldn't apply the change. This would be a larger change, for now let's just fix the crash.
Fixes: https://github.com/scylladb/scylladb/issues/13129Closes#14429
* github.com:scylladb/scylladb:
modification_statement: reject conditional statements with empty clustering key
statements/cas_request: fix crash on empty clustering range in LWT
(cherry picked from commit 49c8c06b1b)
992c50173a