I'll leave an untested WIP-patch against current trunk.

Change descriptions by file:

• +app/views/issues/show.html.erb+
  ** blank (unset) value rendering of issue core attributes (currently a dash [@-@] is rendered for several attributes, while nothing is rendered for other attributes):
  *** consistently render a dash for issue core attributes with a blank (unset) value
  ** spent time rendering (currently the field is not rendered when there's no spent time on the issue):
  *** consistently show the field and render a dash when there's no spent time on the issue (if spent time module is disabled the field and dash won't be rendered)
  ** blank description rendering (currently the field is not rendered when it has a blank [unset] value):
  *** consistently show the field and render a dash for blank description attribute (partly the same as #24015)
  ** full-width custom field rendering (the field is rendered below the attachments section):
  *** render the field above the attachments section (and below the description core attribute)

• +app/helpers/issues_helper.rb+
  ** blank (unset) value rendering of issue core attributes for related issues and subtasks sections (currently nothing is rendered):
  *** consistently render a dash for issue core attributes with a blank (unset) value for related issues and subtasks sections
  ** estimated time rendering (currently the field value always shows the 'Total:' label):
  *** consistently show the field and render a dash when there's no estimated time on the issue, and only show the 'Total:' label with value when it is appropriate
  ** blank (unset) value rendering of half-width issue custom field attributes (currently nothing is rendered for custom field attributes):
  *** consistently render a dash for half-width issue custom field attributes with a blank (unset) value
  ** full-width custom field rendering (currently the field is not rendered when it has a blank [unset] value):
  *** consistently render a dash for full-width issue custom field attributes with a blank (unset) value

Some notes and warnings on the patch:

• contains no tests, doesn't modify existing tests and might break existing tests
• doesn't modify email templates and/or API views

I'd like to get some feedback on the mentioned changes before I spend more time on this patch.