Auto merge of #10099 - Niki4tap:null_fn_lints, r=llogiq

Null fn lints

Adds lints to check for code, that assumes nullable `fn()`.

### Lint examples:

`transmute_null_to_fn`:
```rust
error: transmuting a known null pointer into a function pointer
  --> $DIR/transmute_null_to_fn.rs:9:23
   |
LL |         let _: fn() = std::mem::transmute(std::ptr::null::<()>());
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this transmute results in undefined behavior
   |
   = help: try wrapping your function pointer type in `Option<T>` instead, and using `None` as a null pointer value
```

`fn_null_check`:
```rust
error: function pointer assumed to be nullable, even though it isn't
  --> $DIR/fn_null_check.rs:13:8
   |
LL |     if (fn_ptr as *mut ()).is_null() {}
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: try wrapping your function pointer type in `Option<T>` instead, and using `is_none` to check for null pointer value
```

Closes #1644

---

changelog: Improvement: [`transmuting_null`]: Now detects `const` pointers to all types
[#10099](https://github.com/rust-lang/rust-clippy/pull/10099)
changelog: New lint: [`transmute_null_to_fn`]
[#10099](https://github.com/rust-lang/rust-clippy/pull/10099)
changelog: New lint: [`fn_null_check`]
[#10099](https://github.com/rust-lang/rust-clippy/pull/10099)
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