Moves blank check to after content transformation to catch blank proc and block values

Currently if you have a display like so:

```erb
<%= show_for :normal_attr, if_blank: "Not here" %>
<%= show_for :custom_thingy, if_blank: "Also not here" do %>
  <%= print_out_some_fancy_stuff_sometimes_blank %>
<% end %>
```

The page will display "Normal attr: Not here" but "Custom thingy: " 

This fixes that so blocks and procs also respect the if_blank option.  

Note that the use of `blank?` means any non-whitespace chars, including html will prevent the message from displaying, but this very simple change seems like a huge improvement and it fixes our use case.