Use constant instead of removed const_in_array_repeat_expressions feature

src/allocator/fixed_size_block.rs

@@ -31,8 +31,9 @@ pub struct FixedSizeBlockAllocator {
 impl FixedSizeBlockAllocator {
     /// Creates an empty FixedSizeBlockAllocator.
     pub const fn new() -> Self {
+        const EMPTY: Option<&'static mut ListNode> = None;
         FixedSizeBlockAllocator {
-            list_heads: [None; BLOCK_SIZES.len()],
+            list_heads: [EMPTY; BLOCK_SIZES.len()],
             fallback_allocator: linked_list_allocator::Heap::empty(),
         }
     }