NirShader: don't fail on null constant_buffer

On iris (and probably other platforms too), an empty buffer could be a
null pointer. This is problematic, because even empty slices can't have
a null pointer. When we encounter an empty buffer, send an empty
static slice instead.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28309>

src/gallium/frontends/rusticl/mesa/compiler/nir.rs

@@ -440,7 +440,12 @@ impl NirShader {
     pub fn get_constant_buffer(&self) -> &[u8] {
         unsafe {
             let nir = self.nir.as_ref();
-            slice::from_raw_parts(nir.constant_data.cast(), nir.constant_data_size as usize)
+            // Sometimes, constant_data can be a null pointer if the size is 0
+            if nir.constant_data_size == 0 {
+                &[]
+            } else {
+                slice::from_raw_parts(nir.constant_data.cast(), nir.constant_data_size as usize)
+            }
         }
     }