retain the string "[]" as is, instead of converting it to '[]' (i.e., "")

This addresses #1215.

src/machine/system_calls.rs

@@ -735,9 +735,15 @@ impl MachineState {
     pub fn value_to_str_like(&mut self, value: HeapCellValue) -> Option<AtomOrString> {
         read_heap_cell!(value,
             (HeapCellValueTag::CStr, cstr_atom) => {
-                // avoid allocating a String if possible ...
+                // avoid allocating a String if possible:
+                // We must be careful to preserve the string "[]" as is,
+                // instead of turning it into the atom [], i.e., "".
+                if cstr_atom == atom!("[]") {
+                    Some(AtomOrString::String("[]".to_string()))
+                } else {
                     Some(AtomOrString::Atom(cstr_atom))
                 }
+            }
             (HeapCellValueTag::Atom, (atom, arity)) => {
                 if arity == 0 {
                     // ... likewise.