cex: always show ε/%empty in counterexamples

On a case such as
    %%
    exp
    : empty "a"
    | "a" empty

    empty
    : %empty

we used to display

    warning: shift/reduce conflict on token "a" [-Wcounterexamples]
    Example: • "a"
    Shift derivation
      exp
      ↳ 2: • "a" empty
                 ↳ 2: ε
    Example: • "a"
    Reduce derivation
      exp
      ↳ 1: empty  "a"
           ↳ 3: •

where the shift derivation shows an item "2: empty → ε", with an
explicit "ε", but the reduce derivation shows "3: empty → •", without
"ε".

For consistency, let's always show ε/%empty in rules with an empty
rhs:

    Reduce derivation
      exp
      ↳ 1: empty    "a"
           ↳ 3: ε •

* src/derivation.c (derivation_width, derivation_print_tree_impl):
Always show ε/%empty in counterexamples.
* tests/diagnostics.at: Check that case.
* tests/conflicts.at, tests/counterexample.at: Adjust.

tests/conflicts.at

@@ -1776,7 +1776,7 @@ State 4
       Reduce derivation
         start
         `-> 1: resolved_conflict reported_conflicts 'a'
-                                 `-> 10: .
+                                 `-> 10: %empty .
 
     shift/reduce conflict on token 'a':
        10 reported_conflicts: . %empty
@@ -1790,7 +1790,7 @@ State 4
       Reduce derivation
         start
         `-> 1: resolved_conflict reported_conflicts 'a'
-                                 `-> 10: .
+                                 `-> 10: %empty .
 
 
 
@@ -1999,12 +1999,12 @@ AT_CHECK([[cat input.output | sed -n '/^State 0$/,/^State 1$/p']], 0,
       Example: . 'c'
       First reduce derivation
         start
-        `-> 7: empty_c2  'c'
-               `-> 12: .
+        `-> 7: empty_c2         'c'
+               `-> 12: %empty .
       Second reduce derivation
         start
-        `-> 8: empty_c3  'c'
-               `-> 13: .
+        `-> 8: empty_c3         'c'
+               `-> 13: %empty .