Handle incomplete examples to scenario outlines. Fixes #109.

Gherkin v4.0 is more allowing with respect to incomplete scenario
outlines, therefore Cucumber::Core::Gherkin::AstBuilder need to also be
that.

lib/cucumber/core/gherkin/ast_builder.rb

@@ -271,7 +271,7 @@ class ScenarioOutlineBuilder < Builder
           def initialize(*)
             super
             @step_builders = attributes[:steps].map { |step| OutlineStepBuilder.new(file, step) }
-            @example_builders = attributes[:examples].map { |example| ExamplesTableBuilder.new(file, example) }
+            @example_builders = attributes[:examples] ? attributes[:examples].map { |example| ExamplesTableBuilder.new(file, example) } : []
           end
 
           def result(language)
@@ -305,10 +305,8 @@ class ExamplesTableBuilder < Builder
 
           def initialize(*)
             super
-            @header_builder = HeaderBuilder.new(file, attributes[:table_header])
-            @example_rows_builders = attributes[:table_body].map do |row_attributes|
-              ExampleRowBuilder.new(file, row_attributes)
-            end
+            @header_builder = attributes[:table_header] ? HeaderBuilder.new(file, attributes[:table_header]) : NullHeaderBuilder.new
+            @example_rows_builders = attributes[:table_body] ? attributes[:table_body].map { |row_attributes| ExampleRowBuilder.new(file, row_attributes) } : []
           end
 
           def result(language)
@@ -341,6 +339,16 @@ def result
             end
           end
 
+          class NullHeaderBuilder < Builder
+            def initialize
+              @line = -1 # this inhibits Builder#handle_comments to put any comments in this object. 
+            end
+
+            def result
+              nil
+            end
+          end
+
           def children
             [header_builder] + example_rows_builders
           end

spec/cucumber/core/gherkin/parser_spec.rb

@@ -223,6 +223,29 @@ def feature
 
         end
 
+        context "a Scenario Outline with an empty examples table" do
+          source do
+            feature do
+              scenario_outline 'outline name' do
+                step 'passing <arg>'
+
+                examples do
+                end
+              end
+            end
+          end
+
+          it "creates an examples table node but no example table rows" do
+            allow( visitor ).to receive(:feature).and_yield(visitor)
+            allow( visitor ).to receive(:scenario_outline).and_yield(visitor)
+            allow( visitor ).to receive(:outline_step)
+            expect( visitor ).to receive(:examples_table).and_yield(visitor)
+            expect( visitor ).to receive(:examples_table_row).exactly(0).times
+            feature.describe_to(visitor)
+          end
+
+        end
+
         context "a Scenario Outline with no Examples" do
           source do
             feature(language: 'not-a-language')