Error pruning `IsNull` expressions: Column 'instance_null_count' is declared as non-nullable but contains null values

[alamb]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce

    #[test]
    fn prune_int32_is_null() {
        let (schema, statistics) = int32_setup();

        // Expression "i IS NULL" when there are no null statistics,
        // should all be kept
        let expected_ret = vec![true, true, true, true, true];

        // i IS NULL
        let expr = col("i").is_null();
        let p = PruningPredicate::try_new(expr, schema.clone()).unwrap();
        let result = p.prune(&statistics).unwrap();
        assert_eq!(result, expected_ret);
    }

Actual behavior:

---- physical_optimizer::pruning::tests::prune_int32_is_null stdout ----
thread 'physical_optimizer::pruning::tests::prune_int32_is_null' panicked at 'called `Result::unwrap()` on an `Err` value: Plan("Invalid argument error: Column 'i_null_count' is declared as non-nullable but contains null values")', datafusion/core/src/physical_optimizer/pruning.rs:1776:43
stack backtrace:
   0: rust_begin_unwind
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/panicking.rs:142:14
   2: core::result::unwrap_failed
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/result.rs:1785:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/result.rs:1078:23
   4: datafusion::physical_optimizer::pruning::tests::prune_int32_is_null
             at ./src/physical_optimizer/pruning.rs:1776:22
   5: datafusion::physical_optimizer::pruning::tests::prune_int32_is_null::{{closure}}
             at ./src/physical_optimizer/pruning.rs:1766:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/ops/function.rs:248:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/e092d0b6b43f2de967af0887873151bb1c0b18d3/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Expected behavior
The test should pass (and the Expr::IsNull predicate can be used)

Additional context
We found this while working on IOx

[alamb]

This appears to be a regression of the code introduced in #1595. It means predicate pruning will not happens if an expression contains an IsNull and the statistics don't provide a NullCount so I am not sure how important it is in practice

apache/arrow-rs#1888 (in arrow 17.0.0) where this error comes from started enforcing that the null declaration was correct , introduced by #2778

It looks like there weren't any tests that found it.