Fix error when docsting is just only `@option`

[ksss]

Description

Running yardoc on the following Ruby code will result in an error.

# t.rb
class Foo
  # @option
  def m
  end
end

$ bin/yardoc t.rb
[error]: Unhandled exception in YARD::Handlers::Ruby::MethodHandler:
  in `t.rb`:3:

    3: def m

[error]: NoMethodError: undefined method `length' for nil:NilClass
[error]: Stack trace:
    /Users/yuki.kurihara/src/github.com/ksss/yard/lib/yard/tags/default_factory.rb:147:in `extract_types_and_name_from_text_unstripped'
    /Users/yuki.kurihara/src/github.com/ksss/yard/lib/yard/tags/default_factory.rb:130:in `extract_types_and_name_from_text'
    /Users/yuki.kurihara/src/github.com/ksss/yard/lib/yard/tags/default_factory.rb:79:in `parse_tag_with_types_name_and_default'
    /Users/yuki.kurihara/src/github.com/ksss/yard/lib/yard/tags/default_factory.rb:91:in `parse_tag_with_options'
    /Users/yuki.kurihara/src/github.com/ksss/yard/lib/yard/tags/library.rb:237:in `send_to_factory'
    /Users/yuki.kurihara/src/github.com/ksss/yard/lib/yard/tags/library.rb:169:in `option_tag'

This problem is seen only with @option and not with other major tags.

I presume that this is probably due to the fact that if text is "" in the following code, the result will be [], creating a text == nil condition when retrieved.

https://github.com/lsegal/yard/blob/2d197a381c5d4cc5c55b2c60fff992b31c986361/lib/yard/tags/default_factory.rb#L102

If the Docstring is only "@param", text is still "". To correct the problem, I now treat text as "" when it is nil.

Completed Tasks

• [x] I have read the Contributing Guide.
• [x] The pull request is complete (implemented / written).
• [x] Git commits have been cleaned up (squash WIP / revert commits).
• [x] I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[lsegal]

Sorry for the delay, but thank you for the contribution!