Handle nil case typechecking @!parse blocks (#727)

I hit this on some real-world code, reproduced it with a spec and
fixed it:

```
  1) Solargraph::TypeChecker strict level complains on @!parse blocks too
     Failure/Error:
       stringified_comments[rng.start.line] ||= begin
         buff = associated_comments[rng.start.line]
         buff ? stringify_comment_array(buff) : nil

     NoMethodError:
       undefined method `start' for nil:NilClass

             stringified_comments[rng.start.line] ||= begin
                                     ^^^^^^
     # ./lib/solargraph/source.rb:256:in `comments_for'
     # ./lib/solargraph/type_checker.rb:546:in `block in without_ignored'
     # ./lib/solargraph/type_checker.rb:543:in `reject'
     # ./lib/solargraph/type_checker.rb:543:in `without_ignored'
     # ./lib/solargraph/type_checker.rb:43:in `problems'
     # ./spec/type_checker/levels/strict_spec.rb:15:in `block (3 levels) in <top (required)>'
```

lib/solargraph/type_checker.rb

@@ -551,7 +551,7 @@ def fake_args_for(pin)
     def without_ignored problems
       problems.reject do |problem|
         node = source_map.source.node_at(problem.location.range.start.line, problem.location.range.start.column)
-        source_map.source.comments_for(node)&.include?('@sg-ignore')
+        node && source_map.source.comments_for(node)&.include?('@sg-ignore')
       end
     end
   end

spec/type_checker/levels/strict_spec.rb

@@ -4,6 +4,18 @@ def type_checker(code)
       Solargraph::TypeChecker.load_string(code, 'test.rb', :strict)
     end
 
+    it 'complains on @!parse blocks too' do
+      checker = type_checker(%(
+      # @!parse
+      #   class Foo
+      #     # @return [Bar]
+      #     def baz; end
+      #   end
+      ))
+      expect(checker.problems).to be_one
+      expect(checker.problems.first.message).to include('Unresolved return type Bar for Foo#baz')
+    end
+
     it 'ignores method calls with inferred types' do
       checker = type_checker(%(
         String.new.upcase