rust: return empty slice without using from_raw_parts

As this triggers rustc 1.78
unsafe precondition(s) violated: slice::from_raw_parts requires
the pointer to be aligned and non-null,
and the total size of the slice not to exceed `isize::MAX`

Ticket: 7013
(cherry picked from commit 5dc8dea869)

rust/src/applayer.rs

@@ -58,6 +58,9 @@ impl StreamSlice {
         self.input_len
     }
     pub fn as_slice(&self) -> &[u8] {
+        if self.input.is_null() && self.input_len == 0 {
+            return &[];
+        }
         unsafe { std::slice::from_raw_parts(self.input, self.input_len as usize) }
     }
     pub fn is_empty(&self) -> bool {