Fix crash on empty parentheses () in AST conversion

[ahogappa]

Problem

TypeProf::Core::AST.create_node crashes with NoMethodError on the empty parentheses expression ():

require "typeprof"
TypeProf::Core::Service.new({}).update_rb_file("t.rb", "()")
# => lib/typeprof/core/ast.rb:62:in 'create_node':
#    undefined method 'type' for nil (NoMethodError)
#            case raw_node.type

[sinsoku]

I think it's a good change.

It might also be worth testing a couple more AST paths where DummyNilNode is received by a different parent node:

• [()] — DummyNilNode as an ArrayNode element
• if (); 1; end — DummyNilNode as a condition

Separate issue:
foo(()) (empty parens as a method argument) causes a different error:
RuntimeError: *anonymous_rest

This is unrelated to this PR, but it's worth addressing as a separate issue.

[mame]

Good catch! DummyNilNode has gradually been pulled into more places than I originally intended, so I should probably revisit its design at some point. That said, returning it here is much better than crashing, so I'll go ahead and merge this. Thanks for the fix!