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.

yard/lib/yard/tags/default_factory.rb

Line 102 in 2d197a3

text.strip.split(/\s+/, 2)

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

Completed Tasks

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