Fix crash on empty parentheses () in AST conversion

`AST.create_node` unwraps `:parentheses_node` by assigning
`raw_node = raw_node.body`, but for `()` the body is nil, causing a
NoMethodError on the subsequent `raw_node.type` dispatch. Return
`DummyNilNode` when the body is nil, following the existing convention
for empty syntactic slots that evaluate to nil.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 2 people

lib/typeprof/core/ast.rb

@@ -61,6 +61,7 @@ def self.create_node(raw_node, lenv, use_result = true, allow_meta = false)
       while true
         case raw_node.type
         when :parentheses_node
+          return DummyNilNode.new(lenv.code_range_from_node(raw_node), lenv) if raw_node.body.nil?
           raw_node = raw_node.body
         when :implicit_node
           raw_node = raw_node.value

scenario/misc/parens.rb

@@ -0,0 +1,30 @@
+## update
+def grouping
+  (1 + 2)
+end
+
+def nested
+  ((3))
+end
+
+def empty
+  ()
+end
+
+def with_default(x = ())
+  x
+end
+
+grouping
+nested
+empty
+with_default
+with_default(1)
+
+## assert
+class Object
+  def grouping: -> Integer
+  def nested: -> Integer
+  def empty: -> nil
+  def with_default: (?Integer?) -> Integer?
+end