Add safe navigator to captions check

Encountered an issue on avalon-dev where `@master_file.captions`
returned nil instead of an empty Indexed file so the subsequent check
for content errored. This prevents the HLS manifest from generating,
causing streaming to fail. I think this will be encountered with any
item created after we changed how captions are created/stored. Adding
the safe navigator should take care of the issue.

app/views/master_files/hls_manifest.m3u8.erb

@@ -16,7 +16,7 @@ Unless required by applicable law or agreed to in writing, software distributed
 #EXTM3U
 <% if @master_file.has_captions? %>
 <% captions_list = @master_file.supplemental_file_captions %>
-<% captions_list.append(@master_file.captions) if @master_file.captions.content %>
+<% captions_list.append(@master_file.captions) if @master_file.captions&.content %>
 <% captions_list.each_with_index do |caption, index| %>
 <% label = caption.is_a?(SupplementalFile) ? caption.label : 'English' %>
 <% language = caption.is_a?(SupplementalFile) ? caption.language : 'en' %>