html_node_to_text doesn't emit src-less images

fixes CNVS-13449

test plan
- create a discussion topic with an image without a src attribute
- ensure that notification is sent for the topic

Change-Id: Idb87eaa955089197aaaa2247cc6172d8dd52c6ba
Reviewed-on: https://gerrit.instructure.com/36756
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: David Josse <david@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>

gems/html_text_helper/lib/html_text_helper.rb

@@ -66,8 +66,12 @@ module HtmlTextHelper
              node.children.map{|c| html_node_to_text(c, opts.merge(pre: true))}.join
            when 'img'
              src = node['src']
-             src = URI.join(opts[:base_url], src) if opts[:base_url]
-             node['alt'] ? "[#{node['alt']}](#{src})" : src
+             if src
+               src = URI.join(opts[:base_url], src) if opts[:base_url]
+               node['alt'] ? "[#{node['alt']}](#{src})" : src
+             else
+               ''
+             end
            when 'br'
              "\n"
            else

gems/html_text_helper/spec/html_text_helper_spec.rb

@@ -92,6 +92,10 @@ describe HtmlTextHelper do
       th.html_to_text('<img alt="an image" src="/image.png"').should == "[an image](/image.png)"
     end
 
+    it "should not format src-less images" do
+      th.html_to_text('<img alt="an image">').should == ''
+    end
+
     it "should add base urls to links" do
       th.html_to_text('<a href="/link">Link</a>', base_url: "http://example.com").should == "[Link](http://example.com/link)"
       th.html_to_text('<a href="http://example.org/link">Link</a>', base_url: "http://example.com").should == "[Link](http://example.org/link)"
@@ -102,6 +106,10 @@ describe HtmlTextHelper do
       th.html_to_text('<img src="http://example.org/image.png" />', base_url: "http://example.com").should == "http://example.org/image.png"
     end
 
+    it "should not format src-less images even with base urls" do
+      th.html_to_text('<img alt="an image">', base_url: "http://example.com").should == ''
+    end
+
     it "should format list elements" do
       th.html_to_text("<li>Item 1</li><li>Item 2</li>\n<li>Item 3</li> <li>Item 4\n<li>  Item 5").should == <<EOS.strip
 * Item 1