Rollup merge of #122800 - zachs18:nonnull-slice-is_empty, r=Amanieu

Add `NonNull::<[T]>::is_empty`.

As per #71146 (comment)

I figured this should be fine to be insta-stable (with an FCP), but I can edit if that is not desired.

r? ```@Amanieu```

library/core/src/ptr/non_null.rs

@@ -1575,6 +1575,25 @@ impl<T> NonNull<[T]> {
         self.as_ptr().len()
     }
 
+    /// Returns `true` if the non-null raw slice has a length of 0.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(slice_ptr_is_empty_nonnull)]
+    /// use std::ptr::NonNull;
+    ///
+    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
+    /// assert!(!slice.is_empty());
+    /// ```
+    #[unstable(feature = "slice_ptr_is_empty_nonnull", issue = "71146")]
+    #[rustc_const_unstable(feature = "const_slice_ptr_is_empty_nonnull", issue = "71146")]
+    #[must_use]
+    #[inline]
+    pub const fn is_empty(self) -> bool {
+        self.len() == 0
+    }
+
     /// Returns a non-null pointer to the slice's buffer.
     ///
     /// # Examples