Only unnest parser mlhs nodes when no rest argument is provided

```
(a,), = []

PARSER====================
s(:masgn,
  s(:mlhs,
    s(:mlhs,
      s(:lvasgn, :a))),
  s(:array))
PRISM====================
s(:masgn,
  s(:mlhs,
    s(:lvasgn, :a)),
  s(:array))
```

Author: Earlopain

lib/prism/translation/parser/compiler.rb

@@ -1313,7 +1313,7 @@ def visit_multi_target_node(node)
         def visit_multi_write_node(node)
           elements = multi_target_elements(node)
 
-          if elements.length == 1 && elements.first.is_a?(MultiTargetNode)
+          if elements.length == 1 && elements.first.is_a?(MultiTargetNode) && !node.rest
             elements = multi_target_elements(elements.first)
           end
 

test/prism/fixtures/variables.txt

@@ -45,3 +45,5 @@ Foo = 1, 2
 (a; b; c)
 
 a, (b, c), d = []
+
+(a,), = []